Same as asking a plumber if he can fix your leaking tap next Wednesday or the electrician if he can put that new light fitting up on the 27th etc.
Just ask….are you free next Wednesday for an hour, I would like a massage, hand job and to lick your pussy whilst you rub oil into your tits? You are paying for a service. SP’s prefer clients to be polite and to the point. You won’t offend her, as long as you aren’t asking for something she doesn’t do, and you reduce the chances of being disappointed if she says “Sorry, it’s jam rag week” when you rock up with your tongue all polished and ready to probe.
Basically, the etiquette is be polite, be clear about what you want, turn up looking and smelling fresh, pay her the agreed fee and don’t be a dick head whilst you are there, enjoy. Wash, spin, repeat as many times as you want until either of you has had enough. Simple.
Of course, we all seem to be making the assumption you want to avoid the time of the month. Maybe the inverse is true, in which case the advice above works equally as well if you have vampire tendencies. She can only say no…..which was kind of my point about just asking politely.