If it's freshly pumped in an appropriate breast milk storage bag or bottle it is good for several hours at room temperature, no need to keep it in a thermos. If I was going out for a few hours and didn't want to nurse I just took my pumped milk int the bottle and just kept it in my diaper bag until needed. In a insulated bag with freezer packs directly against it it is good for 24 hours. If you don't know that you will use it within a few hours or on a hotter day it is best to keep it in the fridge or cooler bag and take a thermos 1/2 filled with hot water to re heat the bottle in when needed.
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