Breast milk can remain frozen for up to 6 months or can stay in the fridge up to 3 or 4 days. To thaw from freezer, I would immerse the bag in warm water. I don't think you should boil it (correct me if I'm wrong) and you should never microwave breast milk. Hope that helps somewhat.
never boil your breastmilk. it will destroy a lot of its beneficial qualities.
In the fridge, you can keep it for 3 to 5 days. Everyone's milk is different, so you can sniff it to see if it's still good.
In the freezer- depends on the freezer. If it's part of a freezer/fridge combo that gets opened frequently, the rule is 3-6 months. If it's a chest freezer, 6-12 months.
It is also good at room temp for a few hours. Again, everyone's milk is different, and it depends on the room temperature.
Frozen breast milk- once it's defrosted, use within 24 hours. I found the best way to defrost it was to float the bag in a container of cool water in the fridge. If you're in a rush, you can run it under warm tap water or float it in a container of warm water.
Cold milk can be warmed in a container of warm water, or holding it under warm tap water. Do not microwave it. After it's warm/thawed, you can gently swirl it to mix the layers. You should not shake breastmilk.