As slowly as possible because I’m sick of software “updates” and “improved” hardware screwing things up.
As mentioned earlier, companies cut a lot of corners just to get things out there. They do this at the expense of users actually being able to make use of products. I’m fed up with that, so I’m the opposite of an early adopter. I take various precautions against viruses and whatnot since I deliberately keep my devices and software out of date until absolutely necessary. And I only get new hardware when absolutely necessary.
Changing devices or even updating them frequently creates a situation where I have to work for my technology, not the other way around. Devices and software should work FOR me and make my life easier, not the other way around. So I stick with what works.
I hold onto my tech until the ‘latest, greatest’ upgrade makes it difficult or impossible to operate properly. Or if something breaks. Tech is seldom worth paying to fix if it has fallen out of warranty.
Replace? I’ve still got an Apple IIe that I fire up regularly.
Alright, alright…my current laptop is about 3 years old, and I may replace it sometime next year, with a second-hand model of some sort.
My phone’s about 4 or 5 years old and getting long in the tooth, but no plans to replace it yet.
Laptop: When I buy a new laptop I tend to get whatever the current top-of-the-line is in terms of processor and RAM, then I keep it for a long time. So I’m still happy with my 17" Sony Vaio, which I got in early 2010 and runs Windows 7 Pro. The battery life is now just “good” instead of “excellent,” but these days it’s plugged in 99% of the time anyway and functions as more of a desktop (connected to a monitor, keyboard, mouse, and printer). Sometimes I think about replacing it just to get a new toy – maybe a Surface (I have one for work and hate it as a tablet/laptop, but it could be great as a mostly stationary desktop) – and have even played with configuring a few models online, but I’ll probably keep it for a while longer. It still does everything I want it to, and runs pretty well. And if I don’t go the Surface route, I’m not really sure what I’d want to get (they stopped making Vaios or I’d get another one).
Phone: I’d say the average is 2-3 years. I never keep a phone until it “dies”: I always want something shiny and new well before that point. My current phone (a Samsung Galaxy S7) is only three months old.
Tablet: My 2013 iPad is still great. I don’t see replacing it any time soon.
E-Reader: I tend to buy a new Kindle whenever an update is released. I’m happy with my current Oasis, but whenever a new version comes out with some new feature I’ll get it. 
Smartwatch: I was updating them about once a year when I wore Pebbles (whenever a thinner/prettier one was released), but Pebble is now out of business and in February I switched to the Samsung Gear. I’m currently wearing the heavy and masculine S2 Classic: the minute a smaller one is available, I’ll probably get it. (The S3 is actually slightly bigger than the S2; I’m hoping whenever there’s an S4 it will come in a “girl” size.)
Moore’s law states that transistor density (on iCs) doubles about every 2 years. How that translates into performance is another matter.
I go close to 10 years on my computers, maybe a bit less (pardon the pun). Maybe 5 years on a phone.
I use my “old” hardware until it finally dies an ugly death. I can’t remember ever being happier with the new replacement equipment than I was with the old equipment I was forced to replace (except for no longer having to deal with the death-throe symptoms). I’m currently using my housemate’s donated old Ace desktop, no idea how old it is but it’s running Windows 7 (I only gave up XP when I had to and I installed a little program to stop being nagged about “upgrading” to Windows 10), and I’m using a flip phone I’ve had for maybe 10 years now. Oh, and when I travel (infrequently) I use a Dell laptop that was gifted to me for Christmas 2003. Yes, it still runs, though it has trouble with some websites.
At the very last possible minute - when I absolutely have to, because it no longer does the limited things I want it to do, or a new technology does something I feel I can’t do without.
No idea how old my phone and desktop are - at least 5-7 years. Computer is giving me some issues where I get messages that my old operating system will not support an upgraded site or something. In most instances, I find it preferable to stop using that application, or use the limited version that is still accessible. I like that my phone has a keyboard, instead of touchpad.
Heard on the radio the other day when someone said it actually saves time to upgrade more regularly, because if you wait as long as possible, it is a more involved process, as opposed to continual gradual updates. Makes some sense. But my tech uses are light, and I don’t feel anyt urge to change what still works.
Just bought a new car. Dayum, but that thing is complicated! Not sure I am absolutely sure how to lock/unlock the doors yet! 
When my brother upgrades, I get his old stuff. Works well enough for me.
My cell phone, until a couple of years ago, was a basic pay as you go for urgent calls only model, but my brother convinced me to get a shiny new phone with internet. I suspect I will have that until it dies or I kill it - like the first one that I took swimming after only having it for three months. D’oh!
When it ages to the point of no longer working, either from wearing out, or incompatability, I replace. I’m never going to be an early adopter, but I spend a lot less on my tech than many do.