Agreed, carbon dioxide levels are the limiting factor. Nobody suffocates from lack of oxygen in confined spaces, they suffocate from carbon dioxide poisoning. Cartridges can stretch the air supply, but eventually the CO2 builds to a toxic level. As for compressed air in the sub, the level of pressure constantly rose due to air leaking from the various lines and fittings in the boat. It was virtually impossible to leakproof subs in WW2; I'm not even certain today's subs have eliminated the problem, but I suspect they have, since modern nuclear subs can cruise submerged for months.
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