I would guess 2018 at the very very earliest. But that's just based on the last few years and the Fantasyland expansion timeline. More than likely 2019. Again, I (and no one) has any idea because they haven't released any info.
I'm wondering if they'll open it as they finish certain areas (like Fantasyland) or wait until it's all completed and have a huge grand opening (like CarsLand). I'm guessing the latter because they'll want to make it as showy and impressive as possible since it's something everyone has been speculating and waiting on since the announcement of the purchase. Should be interesting to watch it progress.