IIRC, when I was little, I heard you shouldn't run the fan for very long because it was a fire hazard, but I might be misremembering. Plus I'm in a pretty dry climate, and our home has a painfully low RH level right now, so I wanted to leave it off and let the moisture spread throughout the house. (But would that actually work, or would too much humidity stay in the bathroom?) I tried to google this info, but it's like everyone expects everyone to know how long to run the fan. My family never used those fans when I was growing up though, so I have no frame of reference.
My humidiphobic fiance wants to run the fan for up to eight hours after every shower.... Maybe I should just buy a humidity meter for every room in the house.