This is the one question that nobody can give an answer to. As the ladies here will tell you, we've had some go home after a few weeks and some in for months, we've had some go home at 3 and a half pounds and some at 7. Most of us were told to expect our LO's home on their due dates and treat any time before that as a bonus. There isn'teven a rule of thumb, it really does depend on how well your own LO does each day.
The actual weight usually a factor in whether to discharge, they just need to show a constant gain in weight. As well as this, they will want to see your LO be able to maintain their body temperature, breathe independently and feed without the tube. Some babies do go home with oxygen tubes or feeding tubes but only if this is the only thing they need and further in patient care isn't necessary.
If your LO is feeding from a bottle, and you intend to breastfeed, push the staff to let you put your LO to your breast. Even if he isn't actually feeding from it, it will give him the idea that he is supposed to and will help your supply. The suck reflex doesn't usually come in until 34 weeks but Abby was at the breast from 31 weeks, having a go. It took a while to establish, so the fact we started early was a help. Anytime your LO is in your arms, have a go at feeding.
It is hard, but try to relax about when he is coming home. He will be there before you know it.