For me, it was when the urges were constantly on my mind, becoming more noticeable and less scary. It was when no other sensation could amount to the feeling SH could give me - and the urges were physical, like there was an itch that needed to be scratched and that was the only way to release it. The only resort for me pain - but in truth that is not true at all. That's just my experience, I don't know if there is any real answer for your question. Maybe you can relate.