You can use the pre-made formula for up to 24 hours. Do not store unmixed powder in the refrigerator. You can prepare formula bottles one by one depending on how often your baby is hungry. The formula stays in good condition in the refrigerator for up to 24 hours, so many parents prefer to mix enough formula for an entire day.
Consider how many ounces your baby drinks per feeding session, and then mix a jug of formula or individual bottles for each feeding. You can put the bottle of formula back in the refrigerator if it has been in the bottle container or cold bag for less than 2 hours. However, make sure to use it within 24 hours of its preparation for the first time.