I think about 2 more years. I don't see a Wii successor coming anytime soon, at least to me right now. It's doing well enough and it would not be a smart move to pull the plug. That is unless the console starts to do horrible, which just seems unlikely to me. I believe the Wii will survive another 2 or 3 years to dethrone the PS2.
"The Common Cold Isn't So Common During The Cold"