It really depends on your current draw from your amps, versus the output of your alternator. Stock alternators on a V6 contour are rated at 130 amps. If your constant current draw exceeds that, you'll be slowly draining the battery. Incidentally, even though the alternator is rated for 130 amps, it isn't necessarily putting that out, depending on rpms, especially at idle.
My car, for example, has 3 amplifiers, with a combined total current draw of 120 amps. if I'm cranking the system so it is continuously drawing that kind of power and the car is only idling, My battery is in for a workout. Even worse if I'm running wipers, rear defrost, headlights, etc... While cruising, though, I have no problems while it is at higher RPM's.
If you plan on running a large system with the engine off for extended periods of time, you'll want to consider having a deep cycle battery for those occasions.
