Final answer:
The maximum number of days between inspection intervals for passenger oxygen masks in a non-fixed environment is once per year.
Step-by-step explanation:
The maximum number of days between inspection intervals for passenger oxygen masks used in a non-fixed environment is once per year. This means that the masks need to be inspected and maintained at least once every year.
This requirement is based on the 99th percentile of 1-hour daily maximum concentrations, averaged over 3 years. It ensures that the masks are in proper working condition and can provide oxygen to passengers in the event of an emergency.