Nintendo spaced out the DS and Wii with two years, and I'd imagine they are going to have at least that same gap for the 3DS and the Wii successor. If they reveal a new system, it will be at next year's E3 (and I still think that's a huge "if").
Nintendo may have released the GameCube and GBA in the same year, but back then, they needed a successful product as soon as possible. With how successful Wii is, they've got plenty of time to wait.