Quoting Mason & Carmen's Mommy:" I have heard 4 hrs after warming from a few different sites and people but i dont know, my DD never got ... [snip!] ... i dont know, my DD never got sick but she only gets a bottle once in a while too so I dont know lol, im not very helpful i knS
She is also in Southern Cali not the north like us. So if you think about heating that milk up and then taking it out into the hot weather for 4 hours.. It just kind of sounds nasty. It would likely get warm without heating it up before leaving.