Over the course of time I've personally used the following computers as my primary machines:
1990: Some kind of old Mac (the case made it look like the original Mac) (given to us by a family member) (Died in 1999, mostly used for my parents taxes)
Feb. 1997: Dell XPS M200S, Windows 95, 64MB of RAM, 200MHz Pentium, 4GB HD (Recycled in 2002 because it was slooooow and obsolete)
Nov. 2001: Dell Dimension 8200, Windows XP, 512MB/1.8GHz P4/80GB
(Still in use by my mother as her primary machine, although she is thinking about replacing it with a Macbook) (Slow as a result of a 5.5 year old Windows install, we may reformat in the near future)
Dec. 2002: Dell Dimension 8250, XP, 512MB/2.4GHz P4/120GB
(Still in use by my siblings as primary machine, with a new HD and clean install of XP it runs great for everything but games released in 2005 and on) (Upgraded to a 750GB HD) (May be replaced with a high-ish end Vista PC, 24" iMac, or some kind of laptop in the near future)
Dec. 2006: Macbook 2GHz/2GB/160GB
(I use it every day, love it, and don't think I'll replace it till Summer 2010 at the absolute earliest)
So for me it averages out to about four years. But you need to also consider that we used to have on family computer for everyone except my dad, but people gradually branched off and got new computers while the computer they left behind was still in use. So both the XP machines I've left behind are still being used under my roof.