I just wanted to add a WGs point of view.
I'd say a better strategy would be to say you'll book for half an hour but that you might like to extend if you get along well. I don't know if you guys get a lot of cancellations because the escort has an offer of a longer booking, but if that's the case then she has no reason to bump you if she thinks you might extend anyway, plus it's an incentive for her to give a good service and ensure you get along.
I've never had an incidence where my client has said he only wants half an hour instead of the hour, but if I did, I'd be annoyed-just for the reason that you've let me down for the other half an hour that you never intended to take anyway, so I'd think this is likely to get things off on the wrong foot.
As others have said, if I accepted an hour's booking then when I got there said 'Oh, actually, I have to get up early tomorrow, let's do half an hour instead', I'm sure my client would be annoyed, and rightfully so. It's no different.