DS was 9lb2oz and he was in them about a week and a half. Luckily my sister's friend had a baby girl about 3 weeks after I did so was able to pass them on without any waste. Definitely don't stock up on nappies for a new born. Just a weeks worth to start you off.
DD was in them until 3 months, then size 2 until 6 months and still in size 3 at 10 months but she is a very slender baby.
My mum friends and I all found that we had to try different brands to find a good fit for our babies so that's another reason not to buy loads. Also there are other things like I can't stand the smell of Pampers brand so I'm glad I hadn't been bulk bought them!