The length of a therapy depends on the nature of the difficulty, your level of commitment, and the aims that you have in mind. More severe difficulties generally require longer therapies but this is not the only factor. Short-term therapies can be enough to relieve a clearly defined behaviour or symptom (e.g. fear of flying) or help with the initial shock of a traumatic incident (e.g. hijacking) or loss. They will not properly address less specific difficulties such as trouble sustaining relationships, ongoing sexual abuse, feelings of emptiness, chronic inhibition or wanting to be more mature in one’s approach to life.
I do carry out short-term therapies where a clear and measurable goal exists from the start. Most of my clients choose to continue their sessions for as long as they find them valuable and ideally until they have achieved their goals.