The distance from the front bumper to the rear bumper. Used in drag races as a handicap for starting (I.e. Take 2 lengths ahead, your car has less power.) Or for measuring how much somebody won (I.e he won by 3 lengths!)
I assume you mean how long to charge the battery. You should never let the alternator charge a dead battery if possible. The alternator is designed to keep the battery charged but not to charge a...