Final answer:
Every 24 calendar months, pilots must complete a flight review to legally serve as Pilot in Command, as required by the FAA.
Step-by-step explanation:
The timeframe within which a flight review must be accomplished for it to be valid in allowing a person to act as Pilot in Command is every 24 calendar months. The Federal Aviation Administration (FAA) requires that all pilots must complete a flight review, sometimes referred to as a biennial flight review, every two years to ensure they maintain an acceptable level of proficiency and understanding of current aviation regulations. Failing to complete a flight review in the required timeframe means a pilot cannot legally operate an aircraft as Pilot in Command until a flight review has been successfully completed.