Generally speaking 12V outlets in vehicles automatically switch off either when the vehicle is turned off or after a period of time. Even if this is not the case, the amount of current drawn by it will be very low, so as long as your battery is in good health it would last several days without draining the battery enough to make it difficult to start. However, if you have any doubts about the health of your battery or the drain we would suggest unplugging the charger when you leave your vehicle.