Final answer:
An air operator should retain the operational flight plan and amendments usually for 90 days for post-flight analysis and compliance purposes, but specifics may vary by country.
Step-by-step explanation:
According to international regulations, such as those established by the International Civil Aviation Organization (ICAO), an air operator is required to retain a copy of an operational flight plan, including all amendments, typically for a period of 90 days from the date of operation. The exact duration may vary based on the regulations of the specific country's aviation authority. It's essential for operators to maintain these records as they may be needed for post-flight analysis, incident investigations, and regulatory compliance checks.