Only done it a couple of times but I really enjoy an assisted shower, but if she is in there with me, it has been on paid time.
I often shower (alone) on arrival, and again at the end. Some have made sure I am in the shower 5-10 mins before end so they can still usher me out the door on the hour (which usually rules out a re-visit)
, but mostly I go for the shower when my time is up.
Not sure too many would allow a 20 minute shower on their time
, no matter what your circumstances are.