The 50-minute “therapeutic hour” is an old convention with practical reasons: the therapist needs ten minutes between patients for notes, to reorient, and to avoid running late in a chain all day.
There are exceptions with clinical reasons. Trauma-processing sessions, in EMDR or exposure therapies, often run 80-90 minutes, because a processing sequence that has started cannot be cut off halfway. Couples therapy is longer because there are two people to hear. Sessions for young children are shorter, 30-45 minutes, because their attention will not hold longer.
One aspect that surprises people: the session ends at the agreed time even if you have just reached something important. It sounds harsh and it is deliberate. A frame that stretches every time stops being a frame, and its predictability is part of what makes the space safe. In practice the therapist will note where you left off and you will pick it up there.
If you arrive ten minutes late, the session still ends at the agreed time. Not a punishment — the same logic. The person after you has their hour too.