On the morning of the day before your departure day, you'll get a DME envelope hanging on your doorknob, and in there will be your assigned DME bus pick-up time. The assigned time is about 2.75-to-3.25 hours before your flight time (4 hours for international flights).
The Resort Airline Check-in desk at all resorts is open from 5:00 a.m. to 1:00 p.m. If you are using a "participating airline" and you want to use RAC, you must complete the transaction no fewer than 3 hours prior to flight time. In your case, if you're talking a 9:00 a.m. flight, you should get to the RAC desk as early as possible, to make sure there's no line or other delay which would lock you out of using it due to time constraints. Even if that happens, it's no horrible big deal, you just get your luggage to the DME driver, he/she loads it under the bus for you, then unloads it for you when you arrive at the airport.