My first notebook lasted 3 years, before the brightness of the LCD showed significant deterioration. LCDs on notebooks are built to last the expected useful lifespan of the notebook, so you can expect maybe 3-5 years?
My PowerBook is always on sleep mode when I am not using it. Gives me instant-on when I need it, much like a PDA. Many Mac users don't shut down either, and they have been doing it for years, so I don't think there'll be a problem except for slightly draining the battery.