Here is the kicker . . . The oxgen bottle in the cockpit only lasts 30 minutes. Or thats what we plan on. Of course many factors determine the exact amount. Probably longer. But by regulations at least 30 minutes.
There were 12 crew on board, so minus the two pilots there were a minimum of 10 portable oxygen bottles on board. So theoretically if one flight attendant had all ten bottles in the back, then they might have survived for a long time, provided it wasn't too cold and the cabin didn't go much above 25k. But even if one or more of them survived, they can't gain cockpit access as long as someone is alive in the cockpit.
Of course, if one really wants to play the game, one could say the pilot himself could have gained access to a portable oxygen bottle and last longer.
So since we are playing the theory game, it is conceivable a cabin crew member on a portable oxygen bottle can last as long if not longer than the cockpit crew. IF the pilot removed oxygen from the plane to kill the passengers, he himself only has 30 minutes of oxygen. So at some point if he wanted to remain alive, he would have to re introduce oxygen back into the plane.
IF he re introduces oxygen back into the system, any surviving crewmembers on the portable oxygen bottle could survive for the duration of the flight.
If he chose to kill himself also by not re introducing the oxygen and just let the plane fly forward, a surviving cabin crewmember could have gained access to the cockpit as the young man did in the Helios 522 crash, but not know what to do. And you are correct, run out of oxygen at some point.
In the Helios crash, they found all the portable oxygen bottles had been emptied. So at least more than one cabin crewmember got to them. I figured the surviving cabib crewmember used two bottles for himself to last as long as he did.
The POB's can't be recharged, so once they are gone the flight attendants have no other options unless they can get into the cockpit. But there were at least 10 in the rear cabin (12 crew total on the aircraft, so 10 were in the rear cabin). So, supposing 30 minutes each bottle, or 2/hour (and that's optimistic), one flight attendant could last at least five hours, presuming they had all of the bottles unused, and they could survive the cold when the rear cabin was raised.
The pilots are on a liquid oxygen system, with at least two separate POB's. This oxygen system is completely independent of the system in the back. They have plenty of oxygen for the entire flight (more if there's only one alive in the cockpit). Oxygen availability isn't a limiting factor for the cockpit crewmember(s), unless they leave the cockpit, in which case they must first lower the cabin, or use a POB.
In the Helios crash there was nobody conscious in the cockpit to prevent entry. If the person in the cockpit of MH370 were conscious, they could have prevented any flight attendant(s) from gaining access indefinitely. If there was nobody conscious in the cockpit, a flight attendant could have gained entry, like with the Helios incident, but then of course they'd be in the same situation as the flight attendant was in that case, up a creek without a paddle.
First of all, "liquid oxygen" is NOT an option. Chemical oxygen (most likely, most common) or Gaseous Oxygen (not likely, due to weight and maintenance considerations)
Secondly. Yes, the cockpit oxygen separate from the passengers, however those Bottle(s) (gaseous) are located IN the E and E compartment. NOT the cockpit. Regulatory requirements are for 30 minutes per flight crew member and any jumpseater on deck.
So theoretically, it could be expected a single individual ONLY using cockpit crew oxygen can last ONLY for 90 to 100 minutes or maybe a little bit more.
Boeing does NOT list portable oxygen bottles in the cockpit as part of their emergency equipment. Most likely portable oxygen bottles NOT in the cockpit. However, I will give that a local (Malaysia) regulatory agency "might" require a single bottle, but not likely.
Exactly how many portable oxygen bottles in the cabin, of course would be known to the airline. For our purposes, Boeing lists only 4 with an additional 15 more as an option.
I will try one more time. A single pilot on cockpit oxygen at best can last 100 to 120 minutes on cockpit oxygen. At 25,000 feet for that period of time, he is either dead OR he has turned the packs back on and is using cabin air. If he has kept the pack off, and a flight attendant gets to at least 1 bottle immediately, they can survive for 1 hour per bottle. Thus outlasting the cockpit crewmember.