Why would it mess with our day?
Most of the SPs i see tend to orgasm from receiving oral (yes, i know this is probably in my head, but bear with me), which naturally involves quite a lot of clitoral stimulation. this can result in quite a powerful finish for her and sometimes i am told that she just wants to sleep and relax for the next few hours which she can't do because she has other bookings. some others have told me a good finish leaves them completely uninterested in sex for the rest of the day which similarly affects further bookings. i have only had 2 SPs who told me that they did not want to cum (one was a bizarre experience with a Thai lady) but the other gave this as her reason.
i don't have as much success with penetrative sex, most SPs (seem to) enjoy it, but it can be harder to gauge a genuine finish so i have to go on if they tell me or not (i rarely ask unless i am sure and am teasing them). i booked an SP recently for 2 hours as her first customer of the day who afterwards told me she needed to take the rest of the day off and was cancelling her bookings (whether she actually did this, i do not know).
i have been apologetically banned from visiting some SPs early in the day as a result, which seems like a lot of effort to go through for something which is not a genuine issue. have experienced a similar response in my relationships with civvies (hopefully not because they had a queue of punters unbeknownst to me)

**reading this back, it comes across as quite boastful and that is not my intention. i am genuinely curious if this is just crap SPs say to punters to swell their egos, or if it really is an issue for (some of) them.**
To the OP - i think it is different for every woman, there is no one size fits all