Is it "wrong" to "demand" additional time?
That is answered only by your own personality.
If it were me, I probably wouldn't demand anything. Generally if you have to tell someone how to be professional or decent....they wouldn't get it anyway and would just resent you.

You can be guaranteed that it would be remembered, no repeat visit ever, and definitely noted in the review.

People have the right to act rude...but they also have the responsibility to live with the consequences. Just realize that you found a bad one...and move on.

should be the time you are in the meeting.  She should not be scheduled so tight that she can not absorb a bit of over run.  
I do feel you were scammed but there's not much you could have done except walk out.  I just make a note that is how the provider is & I've yet to return to anyone who's shorted me.
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Same as in the civi world. If you have an SO, than you know ladies usually run late when getting ready to go out.  My ex's always ran late when trying to get out of the house.   My last appointment started about 20 minutes just like yours. However, I politely asked the provider if we could still do an hour during our get to know you conversation.  She smiled and said yes. And she wasn't a clock watcher so the appointment went  over the hour. I've had appointments start as much as 45 minutes late, but I always ask "can we do an hour" at the beginning of the date.  Always ask at the start of the date. Cheating you out of 20 minutes is bad form by this lady. Does it happen often, many of my appointments start late, usually not by much,  but I can't remember ever being cheated on time.  Your date had very poor time management skills.  Please write a review indicate the  way the lady treated you. I would not see her again.  Not sure where you are located, but there are so many great providers out there, that will treat you right. Keep on searching you'll find a great provider eventually.  If this is the worse thing that happens to you while hobbying it ain't so bad.  It's the ncns scenarios that drive me crazy.

The hour starts when you walk in the door. This also assumes that the lady is ready when you get there.  I have been asked to wait a few times with agency girls but in every case she was ready when I got to the room and I got a full hour.

If this was an agency you need to complain to the owner if it was an indie I would re-think future visits and make mention of the issue in your review.

Yes, the hour starts when you arrive in the room/incall/outcall location. Now, if YOU had been 20 min late, then there is always the possibility that your appointment time would not be the full hour... but seen as how, SHE was 20 min late, the hour starts when you finally were able to enter the room. Most of the time, even when a gentlemen is running 15-20 min late, as long as he lets me know, I can still usually accommodate him the full hour.... you should have brought that up with her, but IMO, she probably wouldn't have handled that too well either.