And, need more information...
* Do you have to return (and presumably fuel) a rental car?
* Do you have to check bags?
* Do you have pre-check?
All of those affect the time. As a general ballpark, assuming you need to check bags and don't have precheck, I'd plan on walking through the doors of MCO no later than two hours before departure time. Drive time between WDW and MCO is ~30-45 minutes. I personally would use 45 minutes. So using simple math, leave Disney about 2:45 before departure time.
And more math...
Do you have to check bags? If no, subtract 15 minutes (so leave 2:30 before departure).
Do you have pre-check? If yes, subtract 15 minutes (can possibly be 30, but I like extra "just in case" time).
Do you have to return/fuel a rental car? If yes, add 15 minutes.
No bags to check, with precheck, no rental car to deal with, leave WDW 2:15 before departure.
There are other variables too... mobility issues? Travelling with small kids? Want to get a meal?