It should not be given to a baby less than three months old without a doctor's permission. For older babies or toddlers, follow the directions on the side of the box.
That depends on how heavy and/or old your baby is. The dosing schedule is on the box. The preferred method is using the baby's weight but if you don't know the weight, the age can be used instead.
Always follow the dosing instructions on the side of the motrin bottle. The directions usually go by weight as well as the age of the infant. If what you give does not seem to work call your doctor and they can tell you if you should increase the amount that is given.