MCO can be incredibly busy on weekends. I would plan on an hour travel time, especially if you need to return a rental (and fill the tank). You are driving a route that you are not familiar with, which adds that much more time in reading the map, etc. You really should be to the airport two hours in advance. I'd always rather be there early than get completely stressed over being held up in the security lines. So I'd plan on leaving around 6:00.
Anne