I originally completely over engineered it by tracking the YTD humidity and adjusting for the difference between the current average and last year's and then someone mentioned just putting a second humidity sensor outside the bathroom. So I just do that. If the bathroom humidity spikes about 10% over the humidity outside the bathroom, it turns on and then it turns off at a static number.

It sounds wasteful, but I already had a sensor in my primary bedroom, and the thermostat is right outside the kids bathroom so I didn't have to install anything new.

I then added a cooldown to make sure it doesn't end up in a loop where it keeps turning on and off (that's never actually happened but it seemed like a good idea).

I also use it as a way to control the lights. I use motion detection to turn on the lights, but they used to turn off while someone was in the shower. Now it checks to see if the fan is on and if it is, resets the motion detection timer.