There are several variables in this.
First of all, after you land you will have to make your way to the trams from the gates to the main terminal. This can take 5-10 minutes right there.
Second, are you renting from an on-airport company? If so, you can be to the rental counter 5 minutes after you get to the main terminal. If not, you will have to wait for a shuttle bus to the off-airport place, which could be a few minutes up to 15 minutes, and have a drive of 5-15 minutes depending on which place you rent from, before you get to the counter.
Third, are you in the frequent renters club of the company you're renting from? If so, your time at the counter will be 5 minutes or less, and you probably won't have to wait in line. If not, join it; most of them are free. But if you are not in the frequent renters club, then you will have to wait in the regular line and fill out paperwork before you can go to the garage to get your car.
Lastly, how fast can you walk through the airport? MCOs terminal is very large and can take 5-10 minutes just to walk accross with a wheeled carry-on bag, and the car rental counters and shuttle busses are on Level 1 so you will have to wait for an elevator if you can't manage escalators with your bags.
Will you be stopping at the airport Disney or Universal stores to browse or pick up park passes? This can eat up time, too, although it's a pleasant way to start your vacation and get into the Disney frame of mind.
Basically, minimum time to get behind the wheel after you get off the plane is about 15 minutes, and the max could be up to an hour. But you can cut the time down by joining the frequent renters club (Alamo has Quicksilver, Dollar has Fastlane, Thrifty has Blue Chip, National has Emeradld Club, and so on), which lets you fill out a standard rental contract ahead of your trip and usually gets you into a special line with fewer people in it. Renting from an on-airport company will also save you anywhere from 10-30 minutes since you won't have to wait for a shuttle bus to an off-airport rental facility. And if you go to the MCO web site and look at thier terminal map
, you can get a general idea of where to go once you get off the plane so you won't be lost.
If your return flight is at 5pm, plan on returning your car by 2:30, which will give you 30 minutes, and then you will still get to check-in and go through security by 3pm. the airport recommends that you be there 2 hours before your flight. You don't always need that 2 hours, but better safe than sorry.