The bus theoretically leaves at 9am. They tell you repeatedly you have to be on it at 9 but there is always some jerk who takes too long at breakfast and holds up the bus another 30+ mins to an hour. Important for planning purposes as you should not make the mistake I did of booking early lunch plans thinking 9am is plenty of time to be back for lunch.
Fair enough! The bus actually left before 9am when I took it in the fall because everyone got off early. Disney actually tells you to get off the ship at 7:45am for the 9:00am bus.
So... just don't be the jerk holding up the bus!