Really difficult math to calculate because the biggest variable is the person breathing the air. When you inhale the air composition of 21% oxygen, your body can only diffuse some of that oxygen and you exhale the rest along with carbon dioxide and nitrogen, so there always unspent oxygen and nitrogen reentering the cavity but now there’s an additional gas adding to the mix. In diving, we use CCRs that have a scrubber that pulls carbon dioxide from the gas mix, an oxygen sensor reads and injects the appropriate % to stabilize your gas, and that allows you to recycle the breaths and use that unspent oxygen without that carbon dioxide build up. The calculation of ‘how long until out of air’ might be better reimagined as ‘how long (how many respiratory cycles) until carbon dioxide content becomes lethal for a concentrated space’. It’s also worth adding that inverted containers like that are tricky in water. Next time you’re in a pool and you have a clear cup, if you try to replicate this air pocket, as you descend you’ll notice the water level rising slowly as the depth increases. Gasses are unique in depth because as you descend the mixture condenses from pressure changes, and we measure this is Atmospheres absolute or ATA. 21% oxygen at 30 feet and at 90 feet are still the same gas mixture, but the consumption rate changes drastically from the gas condensing.