This question has been brought to you by Iceland’s mother
She wants to know-suppose Man suddenly becomes extinct, but nothing else (animal, plant, mineral) is affected at all. How long would it take for the Earth to return to its original state? What would be the first major effects? How long would the food chain continue for?
There is no ‘original state’ - if man disappears, some organisms might go extinct (such as those represented only in small captive populations), others might thrive.
All sorts of things might happen to various habitats, including ice ages, droughts and widespread flooding or coastal inundation - this sort of thing was very much the norm before humans came along craving stability.
If you’re asking when there would be no detectable trace of man’s presence here at all, I’m not sure if that would ever happen - the best chance of erasing man’s traces would be crustal recycling - if a chunk of land slips under another plate and gets melted back into magma, it’s a pretty safe bet that nothing much would be left to signify that humans were ever there. I’m not sure that this is scheduled to happen for the entire surface of the Earth before it cools and tectonic processes cease.
In that case, there will be remain some trace of mankind’s presence here, right up until the sun expands and engulfs the Earth.
I’d like to be the first person to make the obligatory plug for George R. Stewart’s Earth Abides. It exactly answers your question, speculating on how long the power system would remain operational, how the roads would deteriorate, which farm animals would survive, and so on.
Well, first Woman takes off those damned high heels. 
This will end up in GD, I suspect. There isn’t a cut-and-dried answer here.
As mentioned, there’s really no way for man to disappear without anything else being affected. Domestic plants and animals, bred for dependence, will certainly have a huge impact with longlasting repercussions. In addition to the sources already mentioned, this issue is dealt with whimsically in Dougal Dixon’s classic “After Man”.
But I don’t worry. By definition of the scenario, I’ll be dead.
“Did you ever think about bears?” 
For a long-range view, you might want to try Evolution by Steven Baxter.
Zev Steinhardt
There will be a run on batteries.
I suspect if man suddenly disappeared the “food chain” (or food web) would continue pretty much as it is now until the sun burns out, or expands until it engulfs the earth, or whatever it is supposed to do.
Homo sapiens is not a irrplacable part of the food web.
I’m imagining monkeys encountering a home, within that home there is a PC, and the screen displays SDMB.
What will they think?
First off, it gets a lot quieter.
The condors don’t make it. Neither does corn-on-the-cob, because the cobs have to be husked and the seeds removed before they can sprout and grow.
There’d be population swings among the wildlife and plant life. Some of the invasive “pest” weeds and animal species would probably spread to their max, and since they’re declared pests often because they can disrupt the local ecology and often wipe out native species, well, those disruptions and wipe-outs would happen. Eventually a new balance would occur, but I’d hate to guess the time it would take.
Critters would be able to move into farmlands, and the farmlands would probably go through phases much like what happens after a wildfire, until they reach their climax phase. It would be a new mix, with plants included in each area that had been introduced by the humans from other continents.
…In California, something like this is happening with mountain lions (cougars, catamounts). A few years ago, it became illegal to hunt them, and between their multiplying and the humans’ spreading into their areas, they are being noticed more and more. Of course they eat deer, wild pig, and any smaller game they can get. They have not maxed out yet, and we’ll see what happens over the next couple of decades. (Digression) What would you bet that the humans change the law again and allow hunting them after all?