Well, that depends. When you are producing plenty of milk your baby may only need to feed off one breast per feeding. The amount of milk you can store in your breasts determines how often a baby will feed and whether baby will want one or both breasts at a feeding.
Dr. Peter Hartman and his team of researchers have determined that women with small breasts and therefore smaller storage capacites will need to feed their babies more frequently. Women with large breasts may or may not have larger storage capacities. Your baby will let you know when she wants to eat and how many 'breasts' she wants each time she feeds.
Let your baby feed as long as she wants on the first breast. If when she comes off, she is still showing feeding cues, you can either put her back to the first breast or offer the second breast. If you know you produce an abundance of milk, you will probably want to put the baby back to the first breast to finish her feed.
I always encourage 'my mothers' to use their own gut feeling. Is your baby hungry? Is your baby fussy? Is your baby really sleepy? In the first 4-6 weeks, always err on the side that your baby is hungry when she shows feeding cues, is fussy, or excessively sleepy.