The life span of a tornado can be anything from a couple of minutes up to an hour, but generally they will last about 15 minutes. Once a tornado reaches its maximum intensity the funnel will start to narrow and then tilt towards the horizontal, causing the width of its path, and therefore its damage, to become smaller. 

A typical thunderstorm, despite bringing the enormous power for lightning, huge hail and heavy rain, will not necessarily spawn a tornado. Fortunately only a small percentage of storms become powerful enough, because they need to last several hours to build up the energy and potential to spin.

Warm, dry air from the high desert regions to the west of Tornado Alley in the US flows at a height of 3000-6000 feet above Tornado Alley. This layer of warm, dry air acts as a lid over the warm, mosit and unstable Gulf air flowing beneath it...this Gulf air is 'capped'.  Late in the day after the sun has heated the surface air even more this capped Gulf air breaks through the dry air above it, and this release is even more violent as the energy has been building all day.

This warm air then rises to incredible heights, even up to the jet stream itself at over 30,000 ft. As the jet stream strips and removes the top layer of the storm, more energy from below is pulled up to replenish it. These conditions then leas to the huge and violent storms of Tornado Alley and the increased probability of destructive and life-threatening tornadoes.