I generally like to inquire days and even weeks in advance with the SP, so I'm not in a hurry to get a response. Once I get my aswers I figure out a day and time and go from there. If it's been a few days and I don't get an answer I just resens my original text to them again and most times I get a response within minutes after a second attempt.
I am not able to fix a timeline on how long I would wait. Minimum 1 hour and sometimes longer depending on how desirable the girl is. One thing I do dislike is getting a response between 1am to 5am which is often typical of a few of the visiting ladies.