Well I have a few reg Gents who I do go over the time with, and that really is a discount. So I don't think if I were you I would ask that of her. If she is a low volume provider, and time is limited as it is with me, I have a certain amount I need each week.  Prebooked appts can allow me to let things get carried away and not worry about the time so much. But that also doesn't mean every time he sees me we are off the clock. Some tend to think once you have done that, it's going to be that way every time.
Just like any other business, it needs to be clear. I hate to talk about finances with my Gentlemen, but when I get put in that possition, I have to.  I guess you could ask, but I don't think it's wise.  You could tell her although you want to see her more often, you can't afford to. If she chooses to give you a discount, she'll let you know.