My son when he was a baby wore newborn size diapers until 1 month old and then at 1 month old he wore size 1 diapers.
Then my baby continued to wear size 1 diapers until he was 4 months old and then at 4 months old he started wearing size 2 diapers.
All babies are different sizes and may need a larger size diaper than others at different ages but usually a baby will wear size 1 diapers for at least a couple months.
Then you'll need to move up to a different size diaper.
If the diaper is beginning to fit too tight then it's time to move up to a bigger size diaper especially if you have to stretch the tabs of the diaper or the diaper is leaving red marks around the babies thighs or waist.