I rarely keep a CPU more than a month or two. But most clients stay with the same CPU for several years, unless they have some way to rationalize a new buy, like a child going off to college who needs a computer.
I put my 2500+ in almost 2 months ago. But I'm swapping it out for a 2700 before the weekend. Then again, I have 12 computers in my house, so I'm not exactly a normal user.