You eat more when you are very hungry, and less when you're not.
Babies do the same thing.
As your baby breastfeeds, pay attention to when he swallows and when he pauses. Sometimes you can hear him swallow, or see his jaw move. As your baby gets full, the pauses become longer. Now is the time to burp him. By allowing him to finish one breast before switching, he will get the rich hindmilk that comes at the end of each feeding.