Rate at which a pendulum bob slows due to air resistance?

I know that "perfect" pendulums would be able to swing forever, unperturbed by air resistance. However, since there is air resistance around us, pendulums (swinging bobs) slow down and move closer and closer to a halt. Say we have a metal sphere of mass m and radius r as the bob, suspended at length l from a point. The bob is allowed to rise to a height $h_{0}$ above its equilibrium position (with the string remaining fully stretched), and is then released. After one swing, the bob reaches a new height $h_{1}$ above equilibrium, and so on, until after swing n, it reaches height $h_{n}$ above equilibrium. At what rate will this damping take place (i.e. how can one theoretically calculate $h_{n}$)? What are the factors that affect it?