So far it has worked out to be every 5 to 6 years.
My previous desktop was purchased in 2003 and consisted of a P4HT 2.8GHz, 512MB ram, and a ATi 9800Pro 256MB for slightly more than $1,000. Since then, I popped in 2GB more of ram as an upgrade ($100) and replaced the video card with a ATi x1950Pro when the 9800Pro just after the 4 year mark ($150).
There is absolutely nothing wrong with the computer. Everything still runs smooth. Games... I can't run max settings anymore, but every game can still run on medium or low settings at a good fps.
The computer I had before that was a AMD K6-2 333MHz with 64MB ram and some no name video card. It also worked beautifully for Internet, MSOffice, and gaming (Diablo II and Starcraft). The upgrade in 2003 was a 10x increase in power. I upgraded because I needed Adobe Acrobat and Illustrator apps for work, and they didn't run on it.
To be honest, the computer I had was fine. The only reason I upgraded because I wanted a Mac and I wanted a laptop. My June 2007 MBP fit the bill perfectly (a bit expensive, but I needed a dedicated video card for some of my work applications. At the time of purchase, *all* laptops with an 8600M GT or faster were $1750-2000+ anyway, so the "Mac Tax" wasn't too bad imo.). Other than a battery gone bad in January (Apple shipped a replacement overnight), I haven't had any issues.