The average adult at rest inhales and exhales something like 7 or 8 liters (about one-fourth of a cubic foot) of air per minute. That totals something like 11,000 liters of air (388 cubic feet) in a day.
The air that is inhaled is about 20-percent oxygen, and the air that is exhaled is about 15-percent oxygen, so about 5-percent of the volume of air is consumed in each breath and converted to carbon dioxide. Therefore, a human being uses about 550 liters of pure oxygen (19 cubic feet) per day.
A person who is exercising obviously uses a lot more oxygen than that. You could determine how much air is moving through your lungs by exhaling into a plastic bag of known volume at each breath and seeing how long it takes to fill the bag.
30 ft structures volume empty is over 7500 cubic ft.
Not counting your extra storage.
.269×60= 16.6 cubic feet per hour
7400/388=19.3 days per person
9.2 days for two people.
The more storage you have the less volume.
It actually takes longer to reach a 15% co2 level when it becomes dangerous.
One oxygen bottle @125 cubic feet. Will give you 6.5 days of oxygen per person.
The oxygen bottle requires no power.