Babies come in all different shapes and sizes so each baby could need size 2 diapers at different ages.
But for most babies they move up to size 2 diapers by the time the baby gets to weighing between 12 to 18 lbs and the baby will generally reach the weight of 12 to 18 lbs by age 7 months.
So your baby will usually be ready to wear a size 2 diaper by age 7 months.
But if your babies size 1 diaper is currently getting too tight then it's time to move up to size 2 diapers.