It depends on if you are an intimate (aka escape) or custom (aka wishes) wedding. I am not sure on the timeline for the intimate. For the custom, it depends on where your reception will be. If you want your reception at a convention hotel / ballroom (for example, the Grand Floridian), then you have to wait until 8 months (although, they may be able to put the wedding pavilion on hold for you at 12 months so no one else can get your time -- I would talk to DFTW). If you are going to do a theme park reception or at a non-convention area (for example, anywhere at Epcot, Ariel's, etc.) then you can book at 12 months for a custom.
We booked Randy Chapman before we had signed our contract w/ Disney. He knows how it works and just told us to call him asap after we got the contract so he would know exactly what time on our date. I know another bride on here was going back and forth between two dates and so paid two deposits to guarantee him for both dates. I would just talk to Misty.
I hope this helps!