I generally go five years between Macs. In my opinion, Macs last much longer than Wintel PC's. I mean this in respect to both hardware and useablility. I have a six year old iMac I have loaded full of RAM, and it's running very smoothly. My mom uses it daily. Sometimes when I use it, I'm amazed at how fast and well it runs. Also, this machine has been on constantly for about three years now, still no problems.
I've owned five Macs from 1994 to present, and only one has given me a hardware problem. My old Power Mac 7100 (~1995) has a dead hard drive, but it lasted quite a while.
In my opinion, a Wintel PC will generally last about three to four years before either something dies, or it just can't handle current software. A Mac can last a good five to six years, perhaps even longer, without problems.