I was tuning my new amp for about 3 hours one evening, and the next morning, I couldn't start my car due to the drain. I had a 300W amp powering an add-on subwoofer to the factory stereo, so it has a higher power drain than normal, so my case is a little different than yours (assuming you have the factory stereo).

Nonetheless, at the drive-in intermission (if they still do that), just to be safe, I would highly recommend that you start the car to get it recharging the battery again.