First I endorse everything TS and M said. Back to the original question. I do not dive with spare air; on recreational dives I dive with the tank on my back. That is either a 72 ft. steel or 80 ft aluminum in most cases. On those dives I am either teaching, or taking pictures or videos. I choose not to be cluttered with the extra gear, so plan my dives accordingly and stick to the dive plan. I have a friend who carries a 3 ft spare air all the time, because it makes him feel better. That's his deal. How long it will last depends upon depth, conditions, the diver's panic level or stress level, and other factors. For commercial, technical, or "real" search and rescue dives, a redundant source is appropriate, often necessary. But for recreational dives with properly maintained equipment, executing a proper dive plan, no need. To each their own.
DivemasterDennis
DivemasterDennis