When baby is hungry!
Babies who are otherwise healthy are pretty good self-regulators; they'll let you know when they need to eat and when they are full.
Most babies will feed every 2 to 3 hours or so, but this is quite variable - and it changes over time. Some newborns seem to do well feeding
every 4 to 5 hours and some seem to need fed every 1 to 2 hours. Discuss your routine with your pediatrician.