I think it is 90 days and you call disney dining.
My 2 DS did it last Aug. I called to 1st time about 30 days out and it was all booked. Checked every few days and thought it wasn't meant to be. But, then I called when we arrived and they had openings 2 days before. So, if you miss out, try last minute because you have to pay in advance and there is a 2 day cancelation policy.
It was cute and they had fun!
Amanda