10 minutes for the shower.
+ 5 for shaving (if between 2 long flights).
+ 5 if toilet inside the shower room.
But, if I see it is very busy for the shower, I will go to the normal toilet (and shave there also), to "free" the shower for other.
I think after 25 minutes the light inside the shower room must "flash" for few second (2 or 3), each minutes. After 30 minutes, the light must begin to "dim".
More than 30 minutes is excessive. Shower are there to use, not to squat...