Once mixed, infant formula is generally good for up to 2 hours at room temperature if untouched. However, once a baby starts drinking from the bottle, it should be used within 1 hour. If the mixed formula is not used within 2 hours at room temperature, it should be discarded or stored in the refrigerator and used within 24 hours if kept cold. Here is a detailed summary:

• Prepared formula at room temperature (77°F or cooler): Up to 2 hours if unopened and unused.
• Once the baby starts drinking: Use within 1 hour or discard.
• Stored in the refrigerator (40°F or colder): Up to 24 hours for prepared formula (if the baby has not started feeding).
• Do not freeze prepared formula.
• After 2 hours at room temperature or after 1 hour post feeding, remaining formula should be discarded to avoid bacterial growth and food safety risks.

This advice applies to powdered formula mixed with water, liquid concentrate, and ready-to-feed formulas with slight variations in timing. The safest practice is to prepare formula as needed and refrigerate unused portions promptly if not feeding immediately.