My system is already over 2 years old so clearly I do. My systems usually last 4 years before needing an upgrade and have historically lasted 5 years before requiring processor changes.

My previous system was used from 2004(early)-2008(late) and only received cooling, gpu and memory expansions in its lifetime.

So far my current system is 2 years old and has had:
a gpu upgrade due to the old one failing
a ram upgrade to expand to 64bit
an os upgrade to win 7
a HDD update to RAID 6 and 12TB more capasity
a HDD update for the OS disk to be a SSD
a recently purchased new SSD to upgrade the previous one that will be used to upgrade my tv computer
a minor cooling upgrade to account for the new HDDs added

I plan to easily be able to continue to use this system for another 2 years. Ultimately I can upgrade my current 965 to one of the 6 core chips of either another extreme edition or to xeon chips. From there I should be able to then get another 2 years or so out of this platform. For a total of at least 6 years. If you jump on the correct platform at the right time you can really make a single system last for many years though isolated upgrades.

Note too that in my case my other computer is on the same architecture so I actually upgrade 2 computers for the price of one at every major update. For example the cpu from my current computer upgrades the tv computer when it is upgraded.