Someone will probably correct me if I'm wrong, but since the cap is wired inline with your amp from the battery, it should begin to charge as soon as you turn on your amp. Remember, it is a capacitor, not a battery, it doesn't hold a charge for an extended period.
At some point after you power off your system, it will discharge, whether there is a draw on it or not. Simply the amount of moisture/humidity in the air will create enough draw to gradually discharge a cap. over time.