Cathy Moore, a certified newborn care specialist, says, “If youve prepared formula at room temperature, it is recommended that you use it within an hour. And if the formula has been sitting out for longer than an hour, you should discard it.” If the formula milk is untouched, you can store it in a refrigerator and use it within 24 hours. Discard the leftover milk if your baby leaves some milk in the bottle after feeding (2).