It is not a frequency. It is a "duty cycle" - the proportion of ON time to total (on+off) time.
The frequency is usually constant (from a few hundred Hertz to thousands or millions of Hz).
The %dimming = %duty-cycle (as a general rule - eg, LEDs, incandescents etc).
EG - if you want 20% brightness, set the PWM to 20% duty cycle.