Unlike most forces, friction (in this case air friction) resists being tied down to a simple formula - it depends on the size, shape and relative speed of the objects or materials involved.
In this simulation the air friction felt by the pendulum is modelled as being proprotional to the surface area of the pendulum bob and proportional to the square of its velocity.
Whether this is accurate depends on the size and shape of the pendulum.
If its motion causes no tubulence, the air friction will be roughly proportional to velocity, if it does cause tubulence it will be roughly proportional to the square of its velocity (or to even higher powers)
HyperPhysics air friction