I didn't use them long with either of my kids. I would recommend swapping them out for larger sizes.
It really just depends on the baby. My daughter was in nb for a month and a half and was 7 lbs at birth.
I'd take some in and exchange for a bigger size. 0-3 months is a good size to have baggy on baby but fine. But I'd keep enough nb clothes for at least 2-4 weeks worth
My son was 6.9 and fit in them for a month or so. You go through like four onsies a day. They spit and poop much.
Size newborn will almost never be used more than a week if you have an average sized baby
It's so hard to know! When you get clothes at showers they are a lot more flexible about the time you have to return and exchange them a lot of times the baby won't even fit in to newborn they need to be in 0-3 months so talk to the store that you did your registry with and find out what their policy is on that?