If you're departing* on a participating airline, you can check in for your flight at the RAC counter at your resort and check your luggage through to your home airport, any time between 9:15 AM (i.e. no more than twelve hours prior to your flight's departure) and 1 or 1:30 PM (time the counter closes).
If you're using another airline, you can leave your luggage with Bell Services/Luggage Assistance.
In either case, you need to be at the Magical Express bus stop fifteen minutes before your scheduled time, bucause that time is when the bus departs from your resort - not when it arrives. If you've left anything - even just your carry-ons - with Bell Services, allow another fifteen minutes to claim it in case there's a line.
*Specified because ALL airlines participate in Magical Express for arrivals.