Wow, ultralight questions…
Variable, as always. MOST people walking in off the street take about 5-10 hours if they can fly regularly, but it’s also a common experience to have training interrupted ever more than in GA and that can slow people down.
It really takes almost the same length of time to be ready to solo an ultralight and a simple light GA aircraft because the skills required overlap so much - you still need to know how to take off, land, turn, and avoid trouble. Ultralights tend to have few instruments and frequently no flaps, no mixture control, no fuel tank switching, and so on, so they are a little simpler than even the venerable Cessna 172 or Piper PA-28. (On such ultralights you use slips instead of flaps on landing, when necessary). Further simplifying matters, ultralights are not permitted at towered airports, and are seldom seen in high traffic areas so that’s one less thing the student has to contend with as well.
One problem is that there are a number of ultralight instructors who will not solo a student in their aircraft - the student has to buy an ultralight of their own, and that’s when they solo. In such cases, a student might accumulate quite a few hours before striking out on their own, but that’s not because they are unready to solo, it’s a lack of opportunity to do so. Whether this is a good way to teach or not is a subject of some controversy in the UL community (let me put it this way - transcripts of the proceedings could only be posted on the BBQ Pit here at the Straight Dope.)
Deinitely less formal than in GA. There’s a LOT more variation in UL instruction than in GA. Some places just teach you how to operate the machinery and turn you loose - no instruction in navigation, airspace, radio operation, regulations, etc. Me, I chose a more formal instruction that included some groundschool, definitely navigation, and flight into GA airports and how to behave politely while doing so. Even so, it covered far less than GA groundschool. Which isn’t necessarially a bad thing - ULs do have far fewer regulations to worry about, can’t fly in any airspace other than G and E without formal permission, and don’t go far very fast, so it takes much longer to get lost while flying (although still possible to do just that). And, if you’re flying your UL where you are legally permitted to do so, within the confines of Part 103 (the part of the regulations that apply to ULs) that’s OK - there really is less you need to know (though extra knowledge never hurts). Which is the point of ULs, really: they’re simple and limited precisely so you don’t have to go through so much in order to use them.
One MAJOR difference between GA and UL is what happens when there’s a ding and the aircraft needs fixing. In GA, you tell the owner or a mechanic and the airplane disappears into a hangar. You hear lots of mysterious noises, and a bit later the airplane comes out and is declared fixed. In the UL world, if the aircraft needs fixing they hand the student a screwdriver and say “tell me when you’re finished. Yell if you need help with anything.”
Well, OK, that is a simplification, but it is true that in ULs the student is expected to learn to be a mechanic as well as a pilot.
Let’s see… since ULs frequently take off and land in a mere 300 feet, making a 600 runway a practical choice, it can mess up your perspective when you’re suddenly in aircraft that require 800-1000 feet to leave the ground or land, and probably are better off with 2500 for either. Even the slowest GA airplane I’ve flown - the C150 - lands at speeds higher than the upper speed limit on an ultralight. It take a little time to adjust. Likewise, dealing with all those extra bits (I chronically forgot the flaps when transitioning to GA). But that’s really not much different than transitioning from, say, fixed gear airplanes with fixed pitch props to complex airplanes with retractable gear and in-flight adjustable props.
Another problem is terminology and background. You might wind up with a student GA pilot who is VERY proficient at actually flying the airplane… but hasn’t a clue about standard airport runway markings, how to use a radio, navaids, the “six pack” of standard instruments, weather terminology… yet is ALSO very proficient at using a GPS. This can really mess with some CFI’s heads. They become so used to students who know little or nothing in all areas, who follow a set syllabus, that they can’t make the adjustment to a student very skilled in some areas and not at all in others. It certainly made finding a CFI I could work with challenging when I moved up to GA. On two occassions I got screamed at “How can you know this and not know that?” Well, because I didn’t need to know that until I moved to GA! Yes, I was very good at forward and side slips because the things I flew didn’t have flaps - and THAT’s why I didn’t know how to use flaps, I hadn’t had an opportunity to use them until I moved to GA. I frequently did know things, but did not know the proper, official terms for those things. I knew what a Hobbes meter was and did, but I didn’t know it was called a Hobbes meter (I first learned it as an “hour meter”) This can make a UL flyer look stupid, ignorant, or willfully obtuse to someone whose sole experience has been a very formal training program. (To be fair, some UL people ARE willfully stupid, ignorant or obtuse… just like in all other areas of aviation)
These problems are even more severe for UL flyers using trikes and powered parachutes moving to fixed wing GA aircraft - at least I only had to switch from a stick to a yoke, for those folks the entire control system is different, operating different controls in a different manner. A trike, for instance, steers by weight shifting, basically altering the location of the center of gravity, and not by moving hinged bits on the wings. Completely different way of going about things. Don’t bother asking me how parachutes steer - I haven’t the first notion. At least when I moved to GA most of what I had to do was worry about scaling up the size of aircraft, not changing to a completely different sort of aircraft.
On the other hand - somehow during my time with ULs I picked up greater than average pre-flight skills and some decent knowledge of practical airplane mechanics. I have and still do catch things on pre-flight other pilots miss (Although I’ll be the first to admit I’m far from perfect - I miss things, too). Maybe it’s because UL’s have a much smaller safety factor and margin for error in some respects - they truly are minimal machines and almost never have a backup anything. Heck, the ones I flew had no electrical system, no radio, no gas gauge… Just a “how high” and a “how fast” indicator - and even those weren’t nearly as reliable as the GA ones.
The thing hardest to get used to? I started out flying open cockpit in a pusher plane, and learned to judge my airspeed by the relative wind. In GA airplanes, enclosed in a cockpit, I found it … um… yes, really almost disorienting not to have that direct feeling of speed. Learning to judge my speed solely by eyeballs took some time.
As for going the other way, from GA to UL… well, a licensed pilot is almost always better versed in navigational methods (outside of GPS, where GA and UL are frequently equals), regulations, and so forth. Frequently, the GA pilot flys much more complex and difficult machinery. And this is where they get tripped up. ULs are simple… deceptively simple. They look like toys. That’s a problem - they’re NOT toys, they’re AIRCRAFT and like any aircraft they can hurt you bad if you don’t approach them with respect. Overconfidence has killed GA pilots moving to ULs. Which is why the UL crowd likes to keep repeating that when Chuck Yeager tried out a UL he took 3 or 4 hours of dual instruction first - and if it’s good enough for Chuck it’s good enough for you.
Honestly, a GA pilot can transition from a fixed wing GA to a fixed wing UL in just an hour or two if they’re willing to approach it as a they would any other transition to a new airplane type. But many do not. Yes, ULs obey the same physical laws, but (at least in aviation) size matters. Compared to GA, a typical UL has far more drag to contend with - so if your engine quits your best glide angle might be ten times the angle you’d need for a GA aircraft in order to maintain a safe airspeed. That difference alone has killed people who weren’t ready for it. ULs typically require more rudder than GA - in fact, some ULs don’t even have ailerons at all so you turn with just the rudders (although taildragger GA pilots don’t seem much troubled by this aspect 
) Even though the ULs I flew were called “fixed wing”, the wings themselves flexed MUCH more than GA wings - and this disturbs a lot of people. When I was learning stalls in a UL my instructor could point out how the disturbed airflow was having an effect on the cloth wingcovers, where you could, indeed, see a faint rippling effect. You don’t get that with aluminum wings. Where you don’t have flaps, GA pilots frequently have to re-learn (or sometimes, really learn for the first time) how to do slips to a landing with sufficient proficiency. ULs usually operate out of grass airstrips - and you have to think about those differently than pavement (and I’ve met a lot of GA pilots who have never flown on anything but pavement). On turf strips you have to worry about the height of the grass, concealed pits and ruts, wet spots, and the like which are not a concern on asphalt or concrete. Well, yes, you do have to worry about puddles on concrete, but you won’t sink into a puddle on pavement - you CAN sink into sufficiently wet, soft mud or grass. This can not only bring you to a halt, it can result in severe damage to your aircraft and maybe you as well, depending on how abrupt the stop. Learning to judge whether it’s safe to land, or how to land while avoiding a problem spot, is an important part of flying ULs.
Actually, I think my UL experience may have served me well when I was forced down into a field - I had landed on rough turf before, I knew how to judge a field, I had a good idea how bumpy the ride would be, and I had no trouble considering landing “off-road”. This is NOT something usually covered in GA training! It IS a common part of UL training.
You also have to think about wind differently - a UL in a 15 knot wind is a lot like flying a C172 in a 30 knot wind (I’ve actually come down from flights in a UL with bruises from the safety harness due to getting knocked around badly). You have to re-think what is safe for flying. My friend down at the local airport who just bought a mini-cub - which, although 3 or 4 times heavier than a legal UL is still a third the weight of anything else he’s flown, in other words, occupying a middle ground between the two - is also learning this. Some of us with the UL and lightplane experience are having to educate him and caution him on the new airplane. Yes, he’s a very competant pilot and easily handles a C172 in a 20 knot wind - he does NOT, however, have the skill to handle that mini-Cub in the same wind. He will, in time, but it’s been VERY eye-opening for him just to taxi the mini-Cub in a brisk wind. When he had to call together a crew of six to get it back to the hangar safely one gusty morning it finally sunk in that yes, he is going to have to use a different scale of “acceptable” for this aircraft.
An ultralight, like any aircraft, is safe when well maintained and flown within its limits. Problem is, most GA pilots don’t know what the UL limits are. It’s not hard to learn, but you have to admit you don’t know in the first place.
Some emergency procedures have significant differences - not in the theory, in the practice. For instance, in both cases if your engine quits you go to best glide speed. For GA airplanes, best glide seems (in my experience) to be less than normal cruise speed, so you pull back a bit on the yoke to reduce speed. In ULs, best glide and best cruise may be one and the same, and in any case, your aircraft has so much drag your airspeed will bleed away on its own very quickly. In ULs (at least the ones I flew) the drill is stick forward to make sure you maintain airspeed, then adjust to best glide. Do this wrong, you could stall your wing without engine power to help you get flying again. And yes, when I moved to GA my UL training had me doing nose-overs during the first couple engine-out drills. It takes a little adjusting. Another difference is in whether or not to re-start the engine. In GA, if your engine quits you are encouraged to re-start if you have the time to do so. In ULs, you are usually discouraged from doing so. It’s important to note that many ULs engines use pull-starts like lawn-mowers, or are hand-propped, which it may not be possible to start while strapped in and flying. Some do have a electric starters and in such a case an attempt at re-start might be appropriate. Might be. Consider also that ULs are frequently flown lower than GA airplanes and you may not have time a lot of time - use the time to set up for a good landing instead of fiddling with a machine that’s already having problems.
Now, that’s just moving from fixed-wing to fixed-wing aircraft. If a GA fixed-wing pilot wanted to fly a UL like a trike (hang glider wing with a motor, basically) that’s a different kettle of fish. Let’s consider controls. Instead of a stick and rudder, or a yoke and rudder, a trike uses a control bar. That’s it. A control bar, a triagular piece of metal tubing (there might be pedals for brakes, but they’re useless in the air). Yeah, that’s a little different. Let’s consider stall recovery. In a fixed wing you push the yoke/stick forward to regain airspeed. In a trike, you PULL BACK on the control bar. In both cases you’re decreasing the angle of attack, it’s just that you’re doing so with completely different motions. In fact, the pitch control on those trikes works entirely “backward” from a stick-and-rudder system. (Trike pilots, of course, insist it is the fixed-wing people who have it all wrong) Needless to say, this can be dangerous in hands trained to a completely different sort of aircraft. Moving from fixed-wing to a trike is more like moving from fixed-wing to rotorcraft than between fixed-wing and fixed-wing. (No, they aren’t rotorcraft, but there are significant differences on par with that transition) It can be done safely - but it must be done properly. Plenty of pilots fly both safely - after they get some training.
I think that gives some idea of the differences. I can’t emphasize enough that all of these flying machines can be flown safely - it really depends much more on the human acting properly than any other factor.