I tend to sell stuff while they still retail some of their original value. As of right now my computer ages are:
MacBook = 1.5 years old
Windows PC = 1 year old
My previous Windows PC was 1 year old as well before I upgraded it, but my PC before that was 5 years old. Prior to that most of my computers were about 2-3 years old before I upgraded them. That's one of the most redeeming features of Windows PC's is that when they get old, all you have to do is replace a few parts and they're like new again. The oldest part in my current system is one of the hard drives that is 6 years old. My current system I expect to last another 2 year before I do any expensive upgrades (mobo, video card, etc..).
As for my MacBook, i'm expecting it to last me another 2.5 years. I have no real intention of replacing it early, since I don't expect it to do everything I use my PC for. I am, however, hoping that Apple will release a headless mid-range Mac with a user upgradeable video card, so that I can ditch my Windows PC once and for all. If not, then i'll just save up and waste money on a Mac Pro that I don't really need.