Night Feedings with Frozen Milk

How do you guys do your night feedings with frozen milk? It takes at least 20 minutes to warm it up and then its only good for an hour. Do you let baby cry for 20 minutes or do you get up at a certain time to warm it while baby is asleep? Im worried how this will work. The only thing i can see is doing formula at night and i really dont want to do that. Could i put the bag of milk in warm water in a tumbler when i go to bed or would it be bad before he wakes up? Please help!