You can start now! But also make a list of things to add at the last minute and keep it taped to the bag. Certain things you use in everyday life such as cell phone chargers, etc. I do recommend buying travel sized toothbrushes and just pack those (don’t forget one for the person staying with you). That way you can simply discard of them when you leave the hospital. The same with toothpaste. This is nice, because it case you end up having to go in straight from a doctors appointment, you can ask someone at home to finish your list and then come join you at the hospital. That way you don’t have to think of a million things at the last minute.