"Is there a rule of thumb for this? "

yes - run it until the moist air is gone. How long that takes is going to depend on the size of your room, the power of the fan, and anything that might obstruct air exchange.

If you run it while showering, you'll keep much of the steam from building up in the first place. From a water management perspective, that is a good thing. If you prefer a steamy shower though, it would be a drawback.

I normally like a nice, hot steamy shower, except after exercising, so I normally don't run the fan during the shower. We have our fan on a timer, so I set it for 30 minutes after my shower. Seems to be enough time to clear the room of steam.

Minutes x fan cf/m = air displacement. On average, in our household 4 showers are taken per day. The fan timers are set for 30 minutes (10 minute shower time plus an additional 20 minutes run time). The shower fans are rated at 110cf/m.

4 x 30 minutes x 110cf/m = 13,200 cubic feet of air displacement. Even if the fans run at only 80% efficiency, air displacement is still 10,560 cubic feet. That's a lot of conditioned (heated or cooled) air pumped out the roof vents. (If you have a 2,500 sq ft home with 8 foot ceilings, half the air in your house is pumped out daily.)

I run it for 30 mins. 10-15 min shower and 15-20 min run time after shower.

DH has started running it for 10 mins after shower only.

This last weekend while cleaning the shower, I noticed the first signs of mildew on the caulk above the tub. It wasn't there a couple weeks before and in my mind that is about when DH changed his fan routine.

I'm sure the fan in the bathroom isn't as large as it should be just because nothing seems to have been done right in this house. Which is why I'd rather run it longer than shorter.

Thanks for the information.

20 minutes............(put it on a timer)

I have an additional question -- what if there is an open window in a small bathroom? My husband believes we should not use the fan when the window is open, as the air coming in from the outside is being drawn out.

Typically, the hot, steamy air rises and the cooler, dry air sinks. That means the moist air goes up and out no matter where the new air is coming from. Of course, a good strong wind will change that equation,but in general, a fan is better at air exchange than a window is.