Are you returning a car? For a domestic flight I'd allot 75-90 minutes for the drive, 45 to return the rental car/shuttle to terminal (depends highly on which terminal, though), 30-45 minutes to check luggage, 30-60 for security. If your flight is at midnight, 7:30 would be a good time. If it's at 11pm, probably 6-6:15, because traffic will be worse.
If you're talking about 9:15 AM, and returning a car, I'd leave no later than 5:30 AM. It's going to be a terrible traffic day with locals going back to work, spring break ending for most schools and lots of people leaving town after their vacation. 5:30 would get you to the rental counter with a good 30 minute buffer to avoid rush hour. Again, depending on the terminal there's a good chance that the security line will be out the door and 90 minutes to 2 hours long until 11am or so on the day after a major holiday.