TBH I do not think there is an exact timetable, for every cruise, as it's depends on resort bookings and what resorts, and these change up and down cruise by cruise. Of course when used the Wonder and Nagic have less people.
If there are high resort bookings there will be more buses, if low, less. They may change and amend routes depending on numbers, i.e. Low resort bookings one bus will drop off at many resorts. If on the other hand one bus has lots going to one place it will be off.
Certainly not waiting until 9 am if the ship is cleared one and a half hours earlier.