so after a shower i leave the bathroom fan on for awhile and i just leave it for about 20 minutes max to get rid of moisture and avoid mold but half the time i hear it turn off with an audible sigh from one of my roommates...so how long is too long for the fan to be running?

Depends. Do you have an outward facing window? If you have an outward facing window in the bathroom you can just open it instead of running the fan. I tend to leave the fan on about 15 minutes. But if your roomie has a problem, talk to him about it, and see how long he wants you to leave it on. If he says 5, then leave it on for 5, unless you are really committed to leaving it on longer.