When hungry. The best time is when junior is hungry, but not too hungry; and also not too sleepy and not too already-upset and worked up. There's no exact clock time that works best for everyone. Follow your baby's lead to learn his or her rhythms and when nursing works best.
ON DEMAND. Feed your child on demand. Let them dictate when they want to be fed and for how long. Usual may be 10 minutes each breast. After about 5 minutes, the fat content of the milk increases and helps to satiate them.