How long can a car battery power a 15 watt stereo via a cigarette lighter?

I have a 15 watt stereo, and planned on plugging it into my car's 12v DC cigarette outlet. I was wondering if this is a good idea, or if it will drain my battery. Also, how long would it be able to last without affecting the battery?|||15 watts? it will barley affect it as long as your driveing it because you should know that the car recharges the battery when it drives|||if you are running it without the car running, it should be fine for a couple of hours. you might want to test it though and make sure your car will still start. it should work, but you just have to be carefull.|||id give it a couple hours but i'd start the car every 45 min, hour, or so. just for a few seconds to keep it charged|||u cool stop tripin