The day of the week would make a difference to me!! We actually left Tampa very early on Easter morning a few years ago and made it to MK in about 55 mins....but that was with us leaving by 7:30am, and there was no traffic at all.
If you are leaving on a weekday??? You are going to have a lot of company on the road. For an 11:55 flight, you want to be at the airport by 10ish...and you need to return your car which is going to add a few minutes.
So, figure on leaving Orlando by 8 as everyone has said. It is 'usually' a 70-80 min ride, depending on traffic. Getting there early is sooooo much better than sitting on the highway, in a traffic jam, stressing!!!!!