Well, if you think the Wii will fail, maybe Nintendo wont make another system. That wont be the case, so the Wii will stay around until the technology to make a better and affordable next-next gen system is available.
Well, I would guess that the project for the new console will have already been started. Well, the research wil have at least. I waill probably be about 4-6 years, depending on technologies.
They prolly started planning for Console 6 now. After all palns for Wii were begnning right after the GC launch. Anyway 5 years, seeing how thats the normal life for a Big N console.
Probibly around four years, not sure about the hand held though, maybe three years for that, or maybe even two for the handheld, but don't worry about the next non-handheld system you have a long wait for that.