Since the Pentium 2 days, the active operation of past motherboards were on a two to three year cycle, with the KT133, KT133A, and Athlon XP days. Now, not so much.
A Core 2 Quad machine with 6GB of RAM I once had (before passing it off to my dad) still is very capable. That machine I expect to give a good run for almost a decade, maybe more now since computing power is not pushing any extreme performance boundaries as of late.
Current Core i7 HTPC and a PC workstation I have definitely would last for years to come, considering both have 32GB of RAM, a 8 threaded CPU, and all modern interconnects - assuming the computer industry isn't being fickle again and changing standards quickly.