This depends on where you store it. At room temperature it will last 5-10 hours. In the refrigerator the milk can be kept fresh for up to 5 days or up 3 months in the freezer. I a deep freezer the breast milk will stay fresh for 3-6 months. After warming the milk it can be used for up to 1 hour. If the milk smells sour it is probably best not to use it.