You can look out for signs of fullness. Their arms fall to their sides and they get sleepy. I call it the Floppy Arm syndrome. You can try moving away and see if your baby rouses to suckle. If your baby unlatches and doesn't want to re-latch, there's a high chance that he has had enough. My baby nurses for about 10 to 15 minutes each time. My friends can nurse their babies for about 30 minutes.
My son used to fall asleep at the boob too when I was breastfeeding. What I normally did was to tickle him, usually under his chin or feet to stir him a little. Sometimes he continues sucking, otherwise if there isn't any response, I'll unlatch him. And don't worry, babies are smart little beings. They'll cry for the boob if they're hungry
I think it's normal because the breasts are supposed to be comforting to babies. I judge whether baby has enough milk based on nappy output since there is no other more reliable way of measurement.
thanks