I would think leaving at 6:30 would be fine. I can't remember ever having much of a line at check-in or at Security at John Wayne (can't say the same for LAX). I would think getting to the airport an hour before your departure would be good enough. Add 1/2 hour travel time and 1/2 hour for car rental checkin, both probably conservative estimates. (I must admit though, I've never rented a car at John Wayne; is the car-rental on-sight? ) You might check the web site for the airline you are flying to see if they have any recommendations for check-in times, but it sounds to me that you've allowed plenty of time.