The early availability depends on several things:
Resort occupancy
Check out time of previous occupant
Housekeeping schedule.
As PamOKW mentioned, if you are willing to accept whatever is ready at the time you arrive, you can sometimes get lucky (but don't plan on getting any requests).
Even if the room isn't ready, you can get your passes and head for a park, take a swim or just explore WDW. The resort will store your luggage, including perishable items, until your room is ready at 4pm.
Good Luck!