Depends on what kind of console it is.
If it's a successor to the 3DS, I could see it coming out earlier. Nintendo don't really have any competition in the handheld market (Sony have basically given up there and smartphones aren't direct competition) so they don't have any other consoles they have to think about keeping their console launches lined up with. It could theoretically come out in 2016/17.
If it's a successor to the Wii U (which I still think is less likely), that raises some awkward questions. Launching a new console in 2016/17 to go up against the PS4 and XO would be tricky because those two consoles won't be as far into their life cycles and they'll hopefully have built up a decent library of really good games by then. Even if the NX is more powerful, it might shoot itself in the foot by coming out too soon. I don't think it is a successor to the Wii U, but whatever the case I think Nintendo should ride the Wii U out for a full cycle and launch its successor around 2018/19. That way it won't be too far ahead of the next Sony/Microsoft home consoles, and in the meantime they can make plenty of money releasing Nintendo-published titles to the loyal fanbase that have bought the console.
