Frozen milk

I was told that breastmilk is only good for 24 hours after its defrosted, is this true? If so, how do you ladies defrost milk that was frozen in a storage bag? So far, i simply let it sit in the fridge until it's ready but is there any faster methods? In case i get called into work and i don't have anything ready? I was also told that heating milk too fast causes it to lose its nutrients?