I'll probably keep mine the full 2 years...which for me is October 2012.
I got lucky with the EVO. It turned out to be one of the most popular phones...so there are lots of accessories for it. There is also a large community of users for this phone as well. It even gets prompt updates from HTC (even if they break the phone :P)
So my next phone will have to be a "star" of sorts. I don't want some limited production phone that only 200 users of this forum have...and there are only like 100 threads on the phone. We'll have to wait and see what the hot thing is when it's time to upgrade. I definately want to be a part of a large support group.
I may even leave Sprint. I went with Sprint due to their plan pricing and the fact that they were the only kid on the block with 4G. By the time I am able to upgrade, LTE will be very common all over, and a switch to Verizon ($20+ extra per month) will be a possibility.