Bruce Hoult wrote:

My recollection of my O2 training is that the rule is 30 minutes over
10,000 ft, or if you go over 12,500 ft at *all*.

No. 12,500' if you exceed it for 30 minutes or 14,000' if you go over that at
all. At 15,000', passengers also have to be on oxygen. Section 91.211.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.