The trip is about 50 minutes. The buses are there and waiting as soon as you get off the ship. Once one is full, it leaves and another pulls in behind it.
You should have enough time to make your flight, if the ship is on time. However, 11:20 is cutting it a little close. You'll want to eat early and leave the ship soon after it clears Customs, usually between 7:30 and 8:00am.
We're usually to the airport and through security by 10:00-10:15am, which would give you enough time for your flight. The process is quicker if you're flying one of the airlines which allows you to check your luggage at the port for the return flight. That way, you can bypass the main check-in at the airport and check in at your gate.