Interesting thread!
For me, I'd say every 2 years I'll replace everything.
My current machine is a white 24" iMac which I got in January 2007. Truthfully, I would have upgraded by now if something really great came along but honestly, the new iMacs aren't all that tempting. The aluminum is nice but it's not enough reason to upgrade. The glossy screen is a step back IMO. I don't game, and for the stuff I do the C2D w/ 2GB ram is very adequate. We have Alu iMacs in the house (including one penryn iMac) and I really can't tell a difference.
However, I am confident that by January 2008 something great will be out in terms of the iMac line - newer design perhaps, noticeably faster, better (not necessarily more) features, etc. At that time, if it's interesting, I'll upgrade. I was thinking I'd get a new computer once Snow Leopard comes out.
Before my iMac, I had an HP mediacenter PC for around 2 years, I think slightly more. it was a P4 w/ 512 mb ram upgraded to 1.5 GB after a year. IT was OK, but the imac is clearly superior.
Before that, I had a Dell dimension desktop for maybe 3-4 years. And before that, I didn't have my own computer
In terms of laptops, same thing - about 2 years, probably a bit more if there's nothing interesting. I had bought a MacBook Pro in August and then the Air came out in January and I liked it so much that I sold my MBP and got the Air. I don't really consider it an "upgrade" per se because I didn't have to pay anything. It was more of a "switch". Lucky for me, Macs have great resale value so even if I can get a few hundred for my 24" iMac in a years time I'll be happy!
Skye12. I'd have to agree with you. I would have sold my 24" iMac by now and upgraded but truthfully I don't really want the new iMac so the upgrade wouldn't be worth it for me. My MacBook Pro was maybe 6 months old max and I sold it for 200$ less than I paid for it. 200$ in 6 months...33$ a month....to have a great machine...how can we complain?