In the military at least for us helo guys its something like 10,000 feet for greater than 3 hours, pilots must don oxygen. For operations above 13,000 feet oxygen must be donned by the whole crew and worn for the duration of the flight till below 13,000.
As for the game, I believe the hypoxia sets in around 20,000 (very slowly,) so set oxygen to 100% when around 20,000 ft.
The effect doesnt go away right away just keep your oxygen at 100% above 20K and you'll slowly watch the darkness fade away.