All babies come in different shapes and sizes so the fit could be different for different babies.
But typically a baby is ready to move up to a size 3 diaper by 9 months of age to 1 year of age and they can sometimes wear size 3 diapers until 18 months of age.
A baby will need to move up a size in diapers when the diaper begins to fit too tight or the diaper is leaking too much.