Hi, there,
It can be hard to know when the breast is empty. Usually the baby's behaviour will be your best guide. Most babies will come off by themselves when they have had enough. After a burp and a nappy change you might offer them the other side. When they are very small one breast may well be enough for them. But as they grow older they will want both sides. Some babies are "all day suckers" and will literally stay there all day - who wouldn't? It's warm, they get a lovely drink of milk and they get to hang out with their favourite girl - mum. Usually if they've been on for 20-30 mins they've had as much milk as they are going to get, but they often stay on for the cuddle!
The best preparation for breastfeeding is to join the Australian Breastfeeding Association and do a breastfeeding class. As in all things, knowledge is power.
Best opf luck with your new addition to the family