ATPL: In accordance with PART-CAT, 10 % of the passengers in a non-pressurized airplane must have an oxygen supply reserve for the entire flight time when the cabin altitude pressure is greater than:
Question 117, Aircraft General Knowledge - Airframe, Systems, Powerplant - ATPL - Airline Transport Pilot license
13000 Ft.
10000 ft but not exceeding 13000 ft minus 30 minutes.