How long will my baby use the step 1-2 diapers (8-15 pounds)?

It depends on the brand & on the baby my son was 9 lbs at birth so he used size one for a few weeks & i remember at 2 months he weighed 14 lbs we had bought him the big box of diapers (huggies brand) it came with around 260-280 diapers cant remember and he didnt even finish them because he had outgrown them the box was size 2 and a big box lasts around 6 wks... & Some babies are allergic to some brands so you shouldn't stock up before you know that your baby is not allergic to the diaper brand you buy =]