I don't think anyone knows what the schedule will be for Paint the Night beyond 2015, just that it will continue on for the foreseeable future. On another site earlier today, I was reading some speculation about the parade, and there are people who think that PtN will eventually settle into a schedule where it runs in the summer, but that the Christmas parade will ultimately return at night for the holidays, and the Cavalcade (or maybe a new Halloween parade) will eventually return to the Halloween party. I think someone even mentioned the possibility of PtN moving over to DCA at some point down the road.
So, there is no telling what will happen or how often Paint the Night will be presented beyond 2015, and especially beyond summer of 2016, but it is likely to continue in some capacity for years.