If I was down there I'd have suggested we all get into a trance-like state to get our breathing to as little required as possible.
Depending on what controls they had available (and most things seemed to be run by angry pixies so maybe not many), it may have been possible to stretch the supply. Normal air is just under 21% oxygen, OSHA requires 19.5% or more, 15% or less and you are at risk of death. I wouldn't be surprised if they ran oxygen a little high during a normal dive as it makes you feel better and more alert. In an emergency, if you could back oxygen off to 19.5 (or maybe even 15.5), everyone inside will automatically slow down and use less oxygen (and feel like shite). At 19.5 they should all be functional. Lower than that and at least some will lose the ability to function (which cuts down on their oxygen use even more but hopefully they are at worst in a coma and not suffering damage). That could drastically extend the time available if they have some control.
EDIT:
From OSHA:
Human beings must breathe oxygen . . . to survive, and begin to suffer adverse health effects when the oxygen level of their breathing air drops below [19.5 percent oxygen]. Below 19.5 percent oxygen . . . , air is considered oxygen-deficient. At concentrations of 16 to 19.5 percent, workers engaged in any form of exertion can rapidly become symptomatic as their tissues fail to obtain the oxygen necessary to function properly (Rom, W., Environmental and Occupational Medicine, 2nd ed.; Little, Brown; Boston, 1992). Increased breathing rates, accelerated heartbeat, and impaired thinking or coordination occur more quickly in an oxygen-deficient environment. Even a momentary loss of coordination may be devastating to a worker if it occurs while the worker is performing a potentially dangerous activity, such as climbing a ladder. Concentrations of 12 to 16 percent oxygen cause tachypnea (increased breathing rates), tachycardia (accelerated heartbeat), and impaired attention, thinking, and coordination (e.g., Ex. 25-4), even in people who are resting.
At oxygen levels of 10 to 14 percent, faulty judgment, intermittent respiration, and exhaustion can be expected even with minimal exertion (Exs. 25-4 and 150). Breathing air containing 6 to 10 percent oxygen results in nausea, vomiting, lethargic movements, and perhaps unconsciousness. Breathing air containing less than 6 percent oxygen produces convulsions, then apnea (cessation of breathing), followed by cardiac standstill. These symptoms occur immediately. Even if a worker survives the hypoxic insult, organs may show evidence of hypoxic damage, which may be irreversible (Exs. 25-4 and 150; also reported in Rom, W. [see reference in previous paragraph]).