[annoying whiney sarcastic voice on]
"But Mom! On TV, the Baywatch babes can rescue an entire submarine crew with just one Spare Air cylinder!!
[annoying voice off]
On a recent trip to Athens Scuba Park, some friends did an experiment with a spare air to see how long it would last. Both were accomplished divers and in good physical shape.
With a full Spare Air cylinder, they were able to get 30 breaths while kneeling on a submerged platform at 20 feet. Then after refilling, they were able to get less than that while swimming at a constant depth.
Their summary is that a Spare Air is best for getting you to the surface in an emergency but at risk of contrcating deco illness since you would be rushing your ascent and not taking a 3 minute safety stop. Spare Air's should never be thought of as a replacement for a "true" redudant air source (aka a pony bottle) that will help you get out of trouble at a deep depth and possible allow you a safety stop as well.