From the Academy of Breastfeeding Medicine: Freshly expressed human milk may be stored safely at room temperature (10–29°C, 50–85°F) for some period of time. Studies suggest different optimal times for room temperature storage because conditions vary greatly in the cleanliness of milk expression technique and the room temperature.
Warmer ambient temperatures are associated with faster growing bacterial counts in stored milk. For room temperatures ranging from 27°C to 32°C (29°C = 85°F), 4 hours may be a reasonable limit. For very clean expressed milk with very low bacterial counts, 6–8 hours at lower room temperatures may be reasonable, but it is best to chill or refrigerate as soon as possible if the milk will not be used during that time.