We are retired so we don't mind waiting behind someone at the dump station.
I have never timed myself, but prior to arriving at the dump station, we fill ten gallon jugs with water, and then after the black tank is empty, I have my wife pour two into the toilet at a time.
It will surprise you how many more little dams get broken and come out and normally after the tenth gallon, the liquid is running clear
I am guessing it probably takes us around 10 minutes.
If I saw several people pulling up behind, I wouldn't do it, and then just do it again the next time
Jack L