This will depend on what you are bringing to therapy. If you are working through a single issue that is affecting you currently, you may only need 6 to 8 sessions. When delving into childhood experiences, which can often be at the core of our adult unhappiness, that can take longer to explore and heal. It’s important to keep an open mind about how long it might take and raise any concerns with your therapist if you have them.
Often clients might come to therapy for a specific issue but as the work unfolds, they become interested in delving deeper. So the honest answer is: how long is a piece of string!