The shortest lifespan Nintendo has yet given a home console is 4½ years, and I don't think they're going to attempt a shorter one.
But alas, that's not what your question is about.
Personally I'm satisfied when I feel the system's potential has been tapped. Nintendo themselves have already done a bang-up job, and several indies have delivered very uniqie experiences, but I'm still hungering to see more crazy stuff.