This is really important. Allowing your baby to finish on one breast before offering the next breast will help maintain your good milk supply and will help your baby get just the right amount of fat and calories that he needs.
Practice the fine art of "baby watching." As your baby feeds at the breast, notice that he naturally takes little pauses or breaks. As these rest periods become longer, you can try gently compressing your breast to see if this causes your baby to start eating again. If your baby has finished on that breast, he will either come off the breast on his own or will fall asleep and stop sucking actively, even once you’ve compressed your breast.
Sometimes babies will do little "butterfly sucks" when they're done, but you won't hear the swallowing or see the deep active sucks that they use when they are eating. At this point it's fine to take your baby off this breast and offer the other side. If baby remains asleep or is not interested in taking the other breast, that's okay — you can start with the other side at the next feeding.