Guidelines for storage:

At room temperature (less than 75 degrees F, 24 degrees C) for up to
4 hours, although the preference is to refrigerate or chill milk
right after it's expressed.
In a refrigerator for 72 hours. You may add small amounts of chilled
milk to a single container over the course of a day.
In a freezer section of a refrigerator/freezer for up to 3 months if
the freezer is separate. The freezer compartment must not be located
in the refrigerated section. Store the milk away from the door of
the freezer and away from the fan of frost-free models.
In a deep freeze (-20 degrees F or less, -29 degrees C or less) for
up to six months.
Thawing and warming milk:

Thaw milk in its container.
Thaw overnight in the refrigerator, under lukewarm running tap water,
or in a pan of lukewarm water.
Do not use hot water or a microwave.
If milk has been frozen and thawed, it can be refrigerated for up to
24 hours. Do not refreeze.
Discard leftover milk from a feeding after 1 hour.