After how long without "knowing" a woman does it become acceptable to pay for it?

If you feel that you must pay for it then go ahead, but, more often than not she doesn't get the cash. You stand to catch a disease, or get arrested, or both, get mugged, get branded a pervert if a friend finds out etc etc. Think on too, she is being used by you, or her pimp, she could be a druggie so the more she get paid the more she jacks up. Leave it, you'll get your legover, just be patient, it's not the end of the world if you don't do it on a regular basis and it won't fall off with lack of use!