You might also want to check out a humidity/temp sensor. You can activate the fan when the humidity gets too high.
I ended up doing this in our upstairs bathrooms - originally had the fan come on when the shower light came on (and then go off 10 mins later or so after light turned off or timed out) but my dear daughter kept turning it off because she didn't like the noise and my wife was annoyed due to some wacky mental control thing - when she wants to turn a light on she doesn't like that a fan comes on too, she wants to decide... sigh.
Anyway using the humidity level to trigger the fan seems to have alleviated the issue - fan runs lowers humidity, FAF (Family Approval Factor) seems okay.
For our powder room with no bath/shower, a simple on for a certain duration when the light turns on then off does the trick. Thinking about adding motion sensors to the mix as well.
