When it is no longer satisfying or when you no longer have the physical ability to session. One of my dearest subs started sessioning in his early 70's and did one session a week with me for close to ten years. His physical health took a turn for the worse, and he had to stop maintaining that schedule. Towards the end of his sessioning days, we went down to once or so a month, and then when he was unable to drive himself or really walk well, I started visiting him in his retirement home every couple of months. By that time, our sessions were extremely light and playful, and he still got such joy out of the smallest kinky acts (just pinching his nipples, slapping his butt, or giving him a GS). I was glad to be able to help him experienc a little bit more joy at the end of his life.