How long did baby last in newborn size diapers? I am looking at stocking up - how many newborn size should I buy?

I bought 2 packets of Newborn size from GOO.N and that was "just nice" before he had to be upgraded to Small size. GOO.N diapers run a little bigger than most other diapers so I guess if you're baby is about 2-3kg, you go could to 3 packets of Newborn sizes because they do poop and pee a heck lot! My son was 4.3kg at birth so he had to be upgraded earlier because of his size