In my experience, if your flight # is listed on the reservation (assuming you're flying in), they keep your car til your flight is in. Twice, I've had it where the new arrival time was PAST when the Avis office is open and they sent an employee back to give me keys both times (when I was only Preferred with them)!!

If you're going to be more than about 2 hours late, though, I'd call them or modify the ressie online to keep them informed.