The earlier the better for traffic even on a weekend. Are you returning a rental car - then add a half hour on to that to fill up with gas, return car and get shuttle to the airport. We normally give ourselves at least an hour to drive to the airport even early in the morning when we expect light traffic. The best thing to do is get up and check google maps when you still have time to leave earlier than you may have planned in case of accidents or higher than normal traffic for that time and day.