Well thanks at least someone jumped in here. Your scenario is not a part of any Boeing QRH that I'm aware of. So if the O2 bottle is at 1800 PSI, how long can it last for 3 crew members at 100%. I think it's surprising long, maybe 3+ hours, but I can't prove that right now. I have gone to a source at Boeing so maybe I will have an answer soon.
Thanks for your comments!