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

Mine is 3 week still in newborn (the newborn brand I use is up to 5kg in weight). Mainly bought Merries (3 pk) but was given free sample of Huggies Ultra and Drypers Touch and provided with free Drypers Wee Wee Dry (1 pk) by the hospital and by my colleagues (1 pk). So total I had 5 pk and used about 2pk