I generally use all the desktops for more than three years. But the longest is still the C2D E6300 (could be wrong about the model) which I had used for four years and three months. However I keep upgrading the GPU every two to two and a half years. I started the C2D with 7600 GT, then moved to 9800 GT.
I am using the i7 950 now, it will be full four years too this December. I started this 9800 GT, then moved to GTX 560 Ti, and now 580 Lightning or something.
So from 2002 till now, I have used four desktops, the Pentium 4 back in 2002, the Pentium 4 HT back in 2006, then the C2D and now the i7
My next upgrade will be on March-April next year.