I'd say I'd aim for at least 3 years, but right now, I don't think I'd even bother trying with regards to this MacBook. I bought the rev A, Core Duo 1.83 GHz MacBook, and it was repaired around 8 times in 1 year. It spent around 10 weeks in repair, which meant that it was in the shop getting fixed 1 day out of every 5 days, on average.
Got a replacement by Apple.....a C2D 2 GHz machine. Much better. Palmrest has cracked twice, but that's it. It's been buggy though. 😱
In total, it has been 2y 5m since my original purchase, although this MacBook is only 1y and 5 months old. 🙂 Not a bad deal.
However, if I had originally purchased a 15" MBP to replace my 12" PB, I would have been a lot happier and would have no problems keeping it for 4 or 5 years. I'll just wait until the MBPs allow for 16 GB of RAM (rather than 4 GB), and maybe 1600x screen resolution before I pull the trigger.