Sorry, I meant PWM period. I calculate the duty cycle on my own as a percentage of the period as you say, but once I know what percentage I want (I evaluate it as a PD controller) I am now just multiplying by the maximum integer, which in my case is 255, which represents one full period for the 8-bit PWM. I want to know what 255 corresponds to in seconds.
As for the device, it's just a bunch of DC motors. I can't use servos because I need both position and force control, so it's PWM for me :-)