Everyone is different with vastly different personalities and struggles to work with that there are no cut and dry time frames for good therapy to be transformative.
That said, for the majority of clients that I work with, it takes about 1-6 sessions before knowing that this approach is right for them. That they can experience that visceral “feel” of change beginning. I tell many clients to give a therapist the benefit of the doubt for at least a month. But within 6-10 sessions, a client should be able to sense if this work is going somewhere. That change will occur with this therapist and this approach.
My main approach: AEDP, had grown out of a brief therapy approach in the 1980’s and 1990’s but grew well beyond brief therapy. It now provides an extremely flexible time frame for change.
I have seen cases of life long issues dissolve in the first or second half of a session. Sometimes right from the start, much more often it takes weeks to months for that to occur.
Although I work towards change as quickly as possible, quick change is not always possible. Some people have traumas and struggles that require a more long term approach.
Although numbers are artificial, about two years is a relative maximum time frame. That is a whole lot of therapy in any case. If no change occurs within 6 months to a year with the client’s engagement in the work, something would be very much missing and a new approach, angle, or therapy would be required.