Number of breaths per cubic foot varies with depth. Consumption rate varies with stress and exertion level. It would be a complicated formula, but maybe it could be searched up on the internet if you were that interested. Some SCUBA divers calculate their average consumption rates in the 0.5 to 0.75 cubic feet per minute rate range and under stress, it could be 3 or 4 times that much.<br><br>As a rule of thumb, the spare air would probably not last much more than a minute or two at shallow depths. May not last even long enough to get to the surface from great depths (say 60+ feet) at safe ascent rates. But that minute or two could be the margin you would need to exit a submerged car or aircraft. I would consider it a very temporary air supply. <br><br>Pony bottles can provide a scuba diver with the ability to ascend from depth with a greater margin of safety.