Final answer:
Aircraft operating above FL250 must carry sufficient oxygen for the crew, with requirements depending on regulations such as the FAR in the United States. The minimum duration is usually for the time needed to safely descend to a lower altitude with adequate atmospheric pressure.
Step-by-step explanation:
When operating an aircraft above FL250 in a pressurized aircraft in an air transport service, the aircraft must carry a sufficient amount of oxygen to supply the crew for an emergency situation. According to aviation regulations, the minimum requirement is dependent on the number of crew members and the expected time needed to descend to a safe altitude where the atmospheric pressure is adequate for normal breathing. Although the specifics can vary by jurisdiction, for example, the Federal Aviation Regulations (FAR) in the United States mandate that at altitudes above 25,000 feet, pressurized aircraft in air transport service must carry enough oxygen to supply each member of the flight crew for an adequate period, which could be up to several hours depending on the flight profile and emergency scenario.
For instance, FAR 121.333(c) indicates that supplemental oxygen for emergency descents must be available for at least the duration it would take to descend to an altitude of 10,000 feet or the altitude at which the atmospheric pressure is sufficient for normal breathing without assistance.