People use and deal with therapy in many different different ways, including when it is time to stop, I think. I have always been very introspective and work in the mental health field myself, but never tried therapy until I was 40 years old. Then I went for ~2 years, with two different Ts (one pretty awful and one decent). It was interesting and I definitely do not regret it, but not really so helpful for most of the issues I wanted to address. I also used it more for fun and as a distraction rather that working on problems, so eventually I stopped because it interfered with my focus on the really important things and it was too expensive as a hobby; I also found it addictive for me. My Ts really did not want me to quit but I don't think they understood how I used it (more as a distraction from tackling my problems) until later when I told them via email. I think for me it was an interesting adventure but I don't feel I want or need to try therapy again in the same format. I would maybe do occasional, short-term consultations.

I imagine that for people who started therapy young and did it for very long periods (say >5 years), it becomes a form of habit and part of life, much like someone who is an athlete and does physical sports in most of their life. Then it is hard to stop because it kinda incorporates into the person and how one lives in general.