The FAA sets stringent inspection requirements on airlines for O2 Generators, Life Vests, Emergency Medical Equipment, and a number of other items aboard an aircraft. All of these items carry expiration dates, which indicate when the item needs to be replaced, recalibrated, or re-approved for service. Depending on the model of the O2 Generator used on an aircraft, the expiration period usually lasts for 10-15 years from the day it was manufactured (not when it was installed), at which point the O2 Generator needs to be replaced.
If the expiration period passes and the O2 Generator hasn't been replaced it's considered a violation by the FAA – a costly challenge that is common throughout the airline industry. An expired O2 Generator will likely continue to function properly, but if the violation is discovered by the FAA it carries fines of $10,000 per flight segment per violation. Each aircraft generally uses one O2 Generator to facilitate oxygen delivery for up to three passengers. That's around 70 O2 Generators for a 757 aircraft. The $10,000 fine multiplies for each O2 Generator in violation of the expiration date, potentially reaching millions of dollars.