Don't freeze milk you plan to use within three to five days because freezing destroys some of the antibodies. You can keep milk that's been frozen and thawed in the refrigerator for up to 24 hours, but don't refreeze it.
Breast milk contains live antibodies that kill bacteria.
Since bacteria is what causes milk to spoil, the antibodies in breast milk will keep it fresh longer than formula or processed milk. But in reality there are no absolute guidelines for breast milk storage. The many variables include the type of freezer you have and how well it works. To help pumped milk stay fresh longer, wash your hands before pumping and make sure the pump and collection bottles are very clean. As with any source of medical or health care information, you should as a preliminary matter satisfy yourself that the credentials and experience of the source are suitable.