If the next Star Wars
film is due for release in 2015, then principal photography will take place in 2014 - Abrams could move on to directing another film in 2015 while supervising post production on Star Wars.
Mind you, he's not being brought into Star Wars
to "reboot" or otherwise reshape it. He's being hired to direct one film, already written, in a series that they're evidently talking to other directors about continuing.