My twins were born at 6lbs 4oz and 7lbs 6oz, we were in the x-smalls within a week (before that it was disposables in the hospital and some preemie prefolds that I tried for a couple days at first). They stayed in them till they were about 4 months, can't recall their weight but I think it was a smidge over the official limit. We did stop snapping the bottom leg snap for a few weeks and that helped.

My sister-in-law used them on her baby for about 2-3 months. They still fit at that point but the baby had outgrown the absorbancy on them (they'd get soaked very quickly) so she started with the smalls...

Remember that build is going to be really important for something like this. How long they last will vary tremendously from one baby to another...

Karen.