What a coincidence! This question was one from an encyclopedia by aboul al-faraj, a 12th century syrian physician. His answer was: "those who lack food should eat when they get it; those who have food should eat when they are hungry." babies tell you they are hungry by becoming very alert and turning their heads to the side of the cheek you stroke gently. Rarely, poor weight gain prompts scheduled feedings.
When he is hungry. Feeding babies "on demand" make for happier babies. During the first year of life babies grow tremendously and have a higher need for energy. Infants will let you know when they are hungry and soon enough they will put themselves on their own schedule.
Baby's schedule. Babies typically feed when they are hungry. You can try to set a schedule, but as soon as you think you have a schedule, your baby will change it! just be sure to wait 2 hours between feeds for breastfed babies, and 3-4 hours between feeds for formula fed babies.
Baby Will Tell You. Babies demonstrate hunger by communicating unhappiness thru fussing. If you can calm your baby down by any means other than feeding, they are not hungry at that time. So if holding them, changing, swaddling, pacifier, movement or such calms them, they are not hungry. Milk is milk and they do not receive it, they will continue to fuss. We can not fool the baby's hunger drive.