My ds lives in Tampa, so when we head over to visit him, we always book a hotel that is very close to the airport...about 5 miles away, if that. It takes us at least 75 mins to get from WDW to the airport area, and that's without much traffic. I would plan on there being traffic on a Sat afternoon! The least amount of time it has taken us was one Easter Sunday morning. We left Tampa (airport area) by 7:15, and got to MK for 8:20ish...I was flying, and there was little to no traffic.
I would plan on leaving WDW by no later than 2:30. THat gets you to TPA around 4, which gives you plenty of time to return the car, get checked in and to the gate. Better to have a bit of extra time to sit around vs rushing and worrying.