Once frozen milk is defrosted the frozen mil will last for up to 7 days after you defrost.
Make sure you store any thawed milk in the refrigerator if not consuming it within 30 minutes to an hour or it can go bad.
Freezing milk is a good way to make the milk last longer than just refrigerating it however just be aware that when you freeze milk the taste of the milk can and will change the longer you freeze it.
Frozen milk will last for up to a year and sometimes longer but the longer you leave the milk in the freezer the less tasty it becomes.
But it will remain safe to drink.
If you freeze milk and then defrost it make sure to drink or use up any thawed milk within 7 days of thawing or defrosting it.