I tend to hang on to systems until the bitter end.
My current rig may have a 4 year old Opteron in it, but IIRC the motherboard is 6ish years old. I tend to buy into a chipset/socket and wring the life out of it with minor upgrades as other people buy the latest greatest and sell off their not all that old stuff.
It's less keeping up with the times, and more planning for minor upgrades in the future as they become dirt cheap to wring the last bits of life out of this investment. ^_^