Babies in NICU have to be able to do certain things before they can go home. Nowadays, there is no minimum weight requirement (thank goodness). Babies need to breath (although some may go home on oxygen). Babies need to hold their temperature like a newborn does (so when a baby can come out of it's special warmer bed and into a normal crib). She should be able to eat (breast, bottle) without stopping her breathing or significant drops in heartrate.
When the neonatologists think the baby might go home, they do a carseat test. Mom & Dad bring in their normal, non-special carseat. The baby sits in the carseat for length of a typical home from hospital trip. If baby struggles to pass this test, NICU will make some special suggestions as far as headgear & stuff.
The general rule of thumb is that baby will come home closer to her due date. They usually say this can happen 2 weeks earlier than due date. And in my past experience, lil' Z came home late Dec, but her due date was early Feb. She weighed between 3-3.5 lbs (she was 2lbs 9 oz at birth).