Formula that's been prepared should be consumed or stored in the refrigerator within 1 hour. If it has been at room temperature for more than 1 hour, throw it away. And if your baby doesn't drink all the formula in the bottle, throw away the unused portion — do not save it for later. Formula may be prepared ahead of time (for up to 24 hours) if you store it in the refrigerator to prevent the formation of bacteria. from: https://kidshealth.org/en/parents/formulafeed-storing.html
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