2016 or 2017. Easy for Abrams to direct both if he chooses - easier still for him to executive produce and call all the creative shots on the next Trek movie.
Disney's plans have been to have the new movies directed by three different people. If they change their minds - no indication of that, yet - and have Abrams direct all three, then he has some commitment to Disney through 2012 or so. If he's only directing the first, of course, he's got a lot of time on his hands in 2015.