In EASA land, this issue is regulated by article CAT.OP.MPA.285 of AIR-OPS.
CAT.OP.MPA.285 Use of supplemental oxygen The commander shall ensure that flight crew members engaged in performing duties essential to the safe operation of an aircraft in flight use supplemental oxygen continuously whenever the cabin altitude exceeds 10 000 ft for a period of more than 30 minutes and whenever the cabin altitude exceeds 13 000 ft.
Of course, airline SOP can define more stringent requirements.