Well I think we can assume that it will come out next year, in October or November.
That would mean that they had a minimum of two years to work on it, and most likely more than that as they talked about having some people work on it while they working on Halo 4, a 'strategy' used by a lot of game companies.
Hopefully most of the pre-development was already finished by the time Halo 4 came out, 2 good years of development should be plenty to make the game. Halo 4 reactions might have made them change some things though, just will have to wait and see.
Of course, nothing ever said Halo 5 will come out next year, they just said Halo will be on Xbox 1.