2-2.5 years, which is quite a long time for me. I used to be a lot more into tinkering, overclocking, etc., but my latest system was built with the intention of actually using it instead of it being in pieces half the time. 😛
Will probably try to get another year or two out of this system and then build something new from scratch (but reuse the case/PSU of course and maybe a few other parts).