Hi Leanne, well I've always had such a good milk supply (nice and big and full) that I just knew when they were empty - they were quite saggy after my daughter finished feeding and she always came off when she has finished. Also, you should never take a bub off the breast, they should always come off themselves. They are very good at letting you know when they've had enough. If they come off one side and still want more then offer the other side. The first side would have been well and truly drained. The first part of the feed the babies get thirst quenching milk which is kind of like skim milk and just basically quenches their thirst. The second part of the feed the babies get hind milk which is the "fat" milk which contains the fat the babies need for putting on weight and growing. It is important that bubs get a good balance of both milk types. Once you've established a good feeding routine, you should be in tune with your breasts and generally know when they're full or empty. Also, after a good long feed it generally takes about a hour for your breasts to fill up again. However, once again everyone is different.
Rachel, 20mth girl + 1 on the way