My current T says that if you are in therapy with the same person for more than 3 years you should probably change due to diminishing returns over time. The thing about leaving is to try to work out if you are running from therapy that threatens to change your status quo ( which after all is why you went there in the first place) because you are scared, or whether the therapy you are getting is not what you need right now.
I've started and stopped many times over the years. Early on it was because I was scared. Later on it was because I had done enough and needed time to assimilate what I had learned. Occasionally it's been because I didn't click with the therapist. Having choice is very empowering.