I think the therapist may know, but I don't think they should tell us when we are done. My therapist said that I would just know, and she was right. I could always find a past wrong, a or present hurt that could have been worked on in therapy, but it would have become a life time oor 10 - 20 years, and I did not want that for me, but it's okay for those that do.
The sign for me was when I no longer grappled 24/7 with wanting to be dead, and when issues came up I dealt with them and my recovery time was much quicker. I no longer needed a therapist, because I could process on my own or reach out for help if I needed it.