ORIGINAL: Roflcopter
The standard average lifetime of a desktop PC is supposed to be 4 years.
I think most companies here write them off in 3.
three years has been the average workstation lifecycle for as long as i've known about it. incidentally, that's when i usually replace my computers. =)
I sort of piggy-back on Mr. Moore, so about every 18 months (couple of months after major introductions) I pick up the old stuff on the cheap. Been doing that for ages. Bleeding edge is not for me, or my wallet. 
on my last two laptops and desktops, i have not once upgraded either of them except for a single extra hard drive that i bought. i never upgrade RAM or CPU, etc. i don't know why that is but i just don't. this time around, i'm seriously thinking of replacing my 3-year old desktop and 4-year old laptop (i got married, thus the delay

) with a single beefy laptop DAW that will also get used for other stuff i do, a bit of gaming, some development, security work, etc.
i usually don't buy top of the line, i typically wait until a technology has established itself and the next step has just come out.