It varies for me. for instance back when I was in grad school, I kept the same system with minimal upgrades for two years. I think during that time I only upgraded the storage (added more) and swapped out the graphics card.
Then some times I upgrade something every couple months. A HDD here, an SSD there. Maybe graphics card. Maybe I'll swap a case for a side-grade. Heck, on Sunday I swapped motherboards because I had a spare and someone offered to buy the one I was using but not the spare (went from Asus P6T to Asus Rampage II Gene). While I swapped the boards I also upgraded the cooling from a Zerotherm Zen with stock fan to a Zalman CNPS10X Flex with dual low RPM Arctic Cooling PWM fans.
So, yeah, some upgrades, some side-grades. Very few new systems from ground up.
I don't always target latest/greatest, however. For instance I'm still using two GTX 285 cards in SLI that I got a while back. However, I'm considering "upgrading" my socket 1366 setup to Sandy Bridge, but I'm in no hurry.
Then again, if I find a hot deal on something...