As Alesia stated it is about 3 hours before your scheduled flight. Buses are scheduled based on the reservations of guests. There is not a routine bus "scheduled route". If you miss your bus, you are most likely on the hook for an expensive cab ride to the airport, as there is not "next scheduled bus" guarantee. If no one else is scheduled to leave the resort for 3 hours, the next bus would be in 3 hours. And even if there was a bus soon, it could very well be full.
So best advice is to be at the DME stop at your resort about 15 minutes before your scheduled pickup
Have Fun