Forever? Well, it's impossible to know that since no one will live forever. I don't think it could last forever, though (the Earth being consumed by the sun can do that to plastic), but they can certainly last a very long time. The Gameboy color, especially, I've noticed, is particularly durable.
well i have the game boy color, sp, ds, advance, and ds light and all of mine work fine but the screen on my old ds does look dimmer. treat them well and they should be fine.