So, for all those craving a rule:
Why, in the 21st century, isn't the rule a prescribed blood oxygen level (or prescribed blood oxygen level for night ops and a prescribed blood oxygen level for day ops, if the science supports the difference)?
A pilot's blood oxygen level is easily and cheaply measured, continuously, in the 21st century. If a pilot can't be trusted to maintain those levels, why can pilots be trusted to use mandatorily-fitted or carried oxygen supplies?