You cannot predict what’s going to happen to you. It depends on how many guests are going to each resort that day. Based on that, they assign the resorts to routes that are not always the same.
For your cruise, some routes might be very popular so they’ll assign multiple buses to it. For those, they’ll start filling the first bus once disembarkation commences, and the bus will leave if it is full. You could be leaving as early as 7:45.
Other routes only have one bus for that day, with a set meeting time of 9 AM. Of course, if everyone booked on it shows up early, it will leave, but don’t count on that. If that is your situation, even if you disembark at 7:30, you could be waiting for the bus to leave until 9.
That being said, you must also consider that the bus will make multiple stops. You are lucky if your resort is first, or you might have to visit 3-4 resorts before yours.
Taking into account all those variables,
DCL recommends not planning anything in a park before 12:30.