Also in the 15-20 min group. That includes a quick, seated review of FT threads as well; though I usually try to do that in the normal bathroom beforehand if I'm waiting (which in turn reduces the time I spend in the shower room as I assume others may be waiting).
I've only gotten the knock once - I forget where. When I came out a few minutes later the lady apologized for rushing me and said she had simply forgotten which rooms were occupied or not.