I am personally not a big fan of very long term therapy (>5 years) because it often focuses attention on the therapy/therapist/dissecting the past too much and distracts from improving everyday life. For me it was interesting: in the beginning when I started therapy, I was very excited about it and thought I would go for a very long time, even if only as a hobby, given my interest in psychology and self examination. It did not end up being the case for me at all, exactly because I realized that I started to use it as a distraction from really improving my life, even in mind, just thinking about therapy too much while neglecting other things. I think it is a very individual thing though, if one finds it genuinely beneficial, then why not go for years or decades? We could see it the same way as regular visits to the doctor or dentist throughout our lives. First maybe to fix some bigger problem, and then just for maintenance. But when it becomes a dependency that shifts the focus from potentially fulfilling endeavors and relationships outside of therapy, when it becomes obsession, when the T encourages staying in spite of the client's dissatisfaction or feeling that it's been enough just to "work through" yet another perceived issue endlessly, then it is probably overdone.

I also feel that the success of therapy is often measured within the therapy, not where things should truly change in the client's everyday reality, especially with Ts that overly focus on the therapist-client relationship.