This depends on many factors: size of bathrooom, how long the shower
was running, how cold it is outside (window condensation), what area
the humid air will flow to if the door is left open, etc. Keep in
mind that while that fan is running, it's sucking conditioned air out
of the house, wasting energy. In the winter, most homes could use the
extra moisture instead of venting it out, provided it flows out of the
bathroom fast enough. Forced air will be better in this regard than
other systems, like baseboard heat. In my house, the master bath is
large and has two doors into different rooms. I never use the fan
after showering.