Apple seem to totally re-design something about every 3 years...
generally, i like to buy a mac product at a good price, and then sell the previous one every year... i always have somethings newish, and i don't loose too much
I do a 4 year upgrade cycle. It seems to be the trend so far. I just upgraded to a Mac Pro after having a G5 PowerMac since 2004 and before that the old G4 from 2000.
As for laptops, I had the first 17" PowerBook circa 2002 then I switched to a 15" MacBook Pro in late 2006 and now I am using the MacBook Air.