Depends on what you do.
If you're a true power user, you reach a point where they benefits of upgrading outweigh the costs of getting a new machine. This is assuming you are getting paid to work on your Mac.
I make my living with my Mac, and I find myself upgrading about every two years. It just doesn't make sense to chug along on an older machine when there's something eight times faster out there... not that the old machine is suddenly nonfunctional, it's just that you can get stuff done better and faster with a new one.
I still see plenty of people working on Macs that are 5+ years old. In most cases, they are also working with older software- Photoshop 5, Quark 4, etc- that doesn't have the same memory and speed requirements that their modern counterparts do. At some point, those people will also have to make the same cost benefit analysis- but I personally believe it's a false economy to put off upgrading for the sake of economy if your livelihood depends on your machine.
My personal upgrade path went something like this:
1991: Mac IIcx
1993: Quadra 700
1994: Mac 7100/80
1996: Umax S900
1999: Blue & White G3
2001: Upgraded BW G3 to a 500 Mhz G4
2002: Dual Gig Quicksilver
So, that's a new Mac or upgrade about once every 2 years on average.