During the winter, when the house was cooler, it was good at room temp
for up to 8 hours. But during the winter, overnight when the room temp
was very cool, my milk was good for at least 10-12 hrs left out. This was a wonderful discovery when I was able to drop pumps. I would pump before bed, and have the bottle ready and waiting for whenever the baby woke during the night- no need to warm or pump.
every person's milk is different. Your best bet is to use the sniff test.