Not that early, but my daughter was born at 32 weeks weighing 3lbs5oz, she stayed 4 weeks. And came home weighing 4lbs.
They will commonly keep baby in the NICU until they would be equal to 35 weeks and at least 5 lbs. That's usually the minimum. If there are more issues they may need to be in for longer but there's no way to tell for sure. Some babies end up in the NICU until they are 6-7 months old.