Coming from wintel land, my machines just tended to metamorphise as time went on. Work provided a laptop, which was refreshed every 3 years.
With my macs, I've had my iBook over 2.5 years now. I thought that would only do 2, but it's ticking along fine. A bit underpowered maybe, but it has it's place now as a web machine. My iMac took over duties, and I think I've hit a sweet spot for the combination. Any serious stuff and I used the iMac - big screen, full size keyboard, hd etc. Just casual stuff, the iBook does admirably, so it's hard to equate a big spend to replace it.
However, if the powerbook line was a bit sweeter, then I could have had a change earlier