Every cruise line will be a bit different when it comes to how quick their disembarkation routine is. With Royal Caribbean, we were off and at the airport by 11:30am. The airport in Ft. Lauderdale is about a 20-30 minute transfer at the most. You can ready a lot of reviews on this site about various cruise lines and you will hear of cruiser's experiences which include how quick they got on and off of the ship.
If you are planning your own flight, to be safe, I would always plan a departure flight after 1:00PM. The airports near cruise lines get busy on cruise days.
I hope that helps.