Hi Kristofer,
the module periodic function (in this case gpio_cam_ctrl_periodic) runs at the frequency you specify via the freq param.
There is no hidden division or something like that.
The maximum frequency is the MODULES_FREQUENCY (defaults to the PERIODIC_FREQUENCY) and must be <= PERIODIC_FREQUENCY.
You set these in your airframe file and it will also show you the values if you compile the firmware with the "print config at build time" checkbox checked.
Consider that the shutter is "pressed/pushed" for DC_SHUTTER_DELAY periodic cycles.