It is unclear what type of milk is being referred to in the question. If the milk is breast milk, it can be stored in clean bottles with screw caps, hard plastic cups that have tight caps, or nursing bags (pre-sterilized bags meant for breast milk) . Breast milk can be stored at room temperature for up to 4 hours, in the refrigerator for up to 4 days, and in the freezer for up to 6 months. If the milk is formula, it should be thrown away after one hour if there is any formula or breastmilk left in the bottle. If the milk is left in the bottle for too long, bacteria from the babys mouth can get into the milk and grow, making the baby sick if they drink the half-finished bottle later.