Freshly expressed breast milk can keep for about 3 to 8 hours depending on the temperature. If at room temperature, the milk can keep for about 6 to 8 hours but if it is warmer like above 30 degrees Celsius, the milk can last for utmost 4 hours. To ensure the milk you pump at work remains fresh until you get home, always keep it in a cooler with ice packs. In the cooler, breastmilk can it can keep fresh for up to 24 hours.