I haven't had a PC long enough to maintain it, but I suspect I'll be upgrading once every 1.5-2 years. I get the feeling that I'm going to upgrade my GPU and CPU at the same time when I decide to make the jump. I have a GTX 760 and a i5-3570k @stock (I only got the k becuause it was on sale at a cheaper price than non-k at the time, I don't feel like trying to OC.) I feel like I'd be bottlenecking a better GPU if I kept the 3570k. I'll probably jump on the GTX 800 (900?) series sometime next summer, if finances allow.
My current gaming rig:
That's one damn fine looking setup.