I had pretty big babies (almost nine pounds each) and I had them both in newborn clothes for probably a month if not more. I really enjoyed the clothes fitting them well to show off their chubby little bodies. But that's just me, I think it looks cuter on babies when they're close fit them well, but not too tight obviously!
My babies wear newborn for 2-3 weeks before moving up. But both me and my husband are very small and we have small babies. Depends on how big your baby is.
I promise that buying newborn sizes are a waste of money. Just buy one or two outfits for pics at the hospital if you like but better to buy size 3 or size 0-3.
My son only fit into newborn for about a week lol every baby is different and like the above posted, every brand is different. I'd get some newborn clothing but just get very little for now.
It depends on your baby. For my children it was a waste 2 of them were able to wear newborn stuff for almost 2 weeks. My oldest was only able to wear one outfit in his hospital picture.
Also different brands are sized differently.