I would say most laptops have a life of 24-36 months. Meaning thats when some new tech or upgrade will come out to warant buying a new laptop. I try to trade mine in around the 24 month period.
it works till the warranty runs out and you gotta pay outta pocket to repair....lol..
as long as the screen isn't smashed/anything seriously wrong it should be usable for years. I have a 1.42Ghz G4 14" iBook and its still OK. I have a 900Mhz iBook G3 that got real beat up and smashed the screen/optical drive and it STILL powers up and can compute and do OK.
Well lets see what the oldest imac and ibook that can run leopard. Well a ibook from october22, 2003 can run leopard with a 1ghz g4 and a imac from february 4th 2003 can run leopard with a 1ghz g4 so it is virtually the same