Major factors include: dining time (early dinner means early breakfast, which means you're off the ship sooner) and/or whether you're going to do self-debarkation. Also know, though, that things can happen. We had late dinner/late breakfast, and anticipated being through customs by 9:30. We were off the ship about 9:15, but we weren't through customs until an hour later. Why? We were on the noro-cruise last April and the line completely stopped. Word is people got sick in line ahead of us. Also, traffic in Miami is a beast. If you're coming in mid-week, plan for it to take awhile longer to get to the airport than it would if you come in on a Sunday.
We had a 1:30 flight out of FLL. I was VERY happy it wasn't any earlier. We got to the airport at 11:30. We had followed Disney's guidelines. My recommendation? Follow their guidelines (on their website).