A psychological therapy is not medicine. Unlike medication wherein the chemical constituents, effective duration and possible side effects of the dosage are known, there are several variables in the course of a therapy which affect its duration quality and timeline.
The individual's state of mind, the environment, family, relatives and other societal factors have a great deal of influence. None of these factors are constant or in any way controllable by either individual or the therapist, let aside the feelings of the patient, which again, are fluidic and keep varying with time. Such fluctuations of the aspects involved not only affect but also influence the overall quality of improvement thereby affecting the progress of the therapy.
One should not have a preconditioned mindset regarding what and how the therapy will be like or what the duration of the therapy should \ would be. Such biases only become the blockades of progression in the course of the therapy and incite a negative outlook.
If this question were to be answered anyway, in black and white, then the duration of the therapy and the quality of progress would only depend upon the individual’s willingness to change, to co-operate, develop tolerance, patience, understanding and most importantly to work upon themselves sincerely.