what kinda stereo? In my other car I have 2 12's, 1000 Watt Amp and some other stuff in there, I can run the subs for quite a while, and i only have a basic battery in there. If you have some big equipment in there, it can depend how long, if its stock, probally a very long time. There is a little thingy you can buy that automatically shuts everything off when the battery gets to the lowest possible voltage to start. So that you dont have to worry about not starting