There is no way to know from cruise to cruise.
For example, this past week at the end of the Panama cruise, people were still leaving the ship after 11 A.M. From what I understood, most of the issue was with the Immigration people inside the terminal.
Once you leave the ship, you go into the luggage holding area (which is huge), find your luggage which can take awhile, then you clear Customs and Imigration, then there is a long line for transporation.
Since I have to fly to the West Coast,I a book a room at an airport hotel and fly out the next day.
Keep in mind that several other ships at both Miami and Fort Lauderdale are also disembarking passengers which can make for long lines at the check-in counters at either airport.