I can't be specific
But as far as I know there is a minimum 02 pressure to allow for dispatch, this is for 3 crew members, when the 02 is below this nominal pressure, the cockpit is restricted to 2 crew members. I do not believe ETOPS duration comes into the equation, I believe the crew would reduce altitude below 10,000' then clear the smoke using the fresh air venting system.
This is not specific to Boeing, but just standard procedure as far as I know, I am a maintenance engineer not a pilot.