I have a bag of frozen breastmilk, how long can it stay out?

👪 5/6/2016🚜👣
I'm going to the ocean today and I want to have some milk in case we get stuck in traffic & he gets hungry on the way back home. How many hours from when I take it out of the freezer is it still good for? Is the milk that defrosts in the bag okay for longer than 6 hours because it's still next to milk that is frozen?