I prefer 3-5 days notice, mainly because my availability is limited, and I hate letting people down. It's usually a lot easier to get your desired timeslot if you request it a week out than if you try contacting me the same day. I do get last minute cancellations from time to time, but I'm more likely to fill that slot with someone I know and trust than some guy who is contacting me for the first time.
Ultimately, every woman is different, and schedules may not align properly for certain couplings. It is nice to know that, given the variety offered here in Houston, there are ideal partners out there to suit certain needs.