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

One of my friends had a relatively small baby, only 2.3kg when they brought her home. She tried Mamy Poko and felt the the fit was about right. She soon switch to S size after her baby grew to about 3.5kg (after abut 2 weeks). Her suggestion is to buy the newborn size diapers sparingly (based on demand and growth rate) but stock up on the different sizes once you find a brand that suits your baby.