what is the lifespan of a dragonfly?

Answer 1

usually no more than four weeks

http://experts.about.com/q/Zoology-1354/Dragonflys-1.htm

Answer 2

Depending on the species of dragonfly, the lifespan can be as short as six months or as long as six or seven years.

Most of a dragonfly's life is spent in the larval stage, living in water, breathing through gills, and feeding on smaller aquatic animals -- sometimes even tadpoles and small fish.

Once they mature and begin to fly, they live for about four more months.

By the way, they don't bite people unless they're captured, and the adults eat mosquitoes and other potentially harmful insects.