This is how you should safely store expressed breast milk

Before expressing or pumping milk, ensure that you have washed your hands.

Use breast milk storage bags or clean food-grade containers with tight fitting lids made of glass or plastic to store expressed breast milk.
Avoid bottles with the recycle symbol number 7, which indicates that the container may be made of a BPA-containing plastic.
Never store breast milk in disposable bottle liners or plastic bags that are not intended for storing breast milk.
Freshly expressed or pumped milk can be stored:
At room temperature (77°F or colder) for up to 4 hours.
In the refrigerator for up to 4 days.
In the freezer for about 6 months is best; up to 12 months is acceptable. Although freezing keeps food safe almost indefinitely, recommended storage times are important to follow for best quality.
Storage tips:

Clearly label the breast milk with the date it was expressed.

Do not store breast milk in the door of the refrigerator or freezer. This will help protect the breast milk from temperature changes from the door opening and closing.
If you don’t think you will use freshly expressed breast milk within 4 days, freeze it right away. This will help to protect the quality of the breast milk.
Freeze breast milk in small amounts of 2 to 4 ounces (or the amount that will be offered at one feeding) to avoid wasting breast milk that might not be finished.
When freezing breast milk, leave about an inch of space at the top of the container because breast milk expands as it freezes.
If you will be delivering breast milk to a childcare provider, clearly label the container with the child’s name and talk to your childcare provider about other requirements they might have for labeling and storing breast milk.
Breast milk can be stored in an insulated cooler bag with frozen ice packs for up to 24 hours when you are traveling. Once you arrive at your destination, milk should be used right away, stored in the refrigerator, or frozen.

Source: CDC

httpss://www.cdc.gov/breastfeeding/recommendations/handling_breastmilk.htm#:~:text=Freshly%20expressed%20or%20pumped%20milk,to%2012%20months%20is%20acceptable.