A batter head is the drum head that will be struck by the stick (top head). A resonant head will be attached to the bottom side of the drum (the front side of the kick drum), adding volume and tone to the drum. Obviously, batter heads will need to be replaced much more often than resonant heads. It's good to change your resonant head every third time the batter heads are changed. Depending on how often you play will determine how often you need to change your heads. It can also vary with how hard you play and what type of heads you're using. In most cases the average player will replace their batter heads every 3 months. This for the player that is averaging 10-15 hours a week. You should be able to tell when the head needs to be replaced. The tone and response will completely change.