This article is from the Breast Pumps FAQ, by firstname.lastname@example.org (Beth Weiss) with numerous contributions by others.
Milk will keep for 6 hours at room temperature, 48 hours
in the fridge, two weeks in a little top-of-the-fridge freezer,
and six months in a big deep freeze. Date each package and
use the oldest first.
You prorate exposure: that is, if the milk was at room temperature
for two hours coming home, it has used 1/3 of it's life and can
only have 32 hours in the fridge now.
Adding More Milk to Already Frozen Milk
Also, you can "top up" a bottle of frozen milk under the followng
circumstances: you must have more in the bottle than you
are adding, (eg adding 1/2 ounce to 4 ounces) and you must
add it cold -- chill it in the fridge first. For example, you can
keep a bottle going in the freezer: pump an ounce or two and use
1/2 ounce or so at a time to use to make up rice cereal.
Whatever isn't used put in the fridge for an hour or so
and then add to the bottle in the freezer.
BTW, I was always told to defrost it under COLD running water,
then warm it up with hot.