There is no hard and fast rule. The length of a course of therapy is entirely dependent upon the individual/couple. However, in therapy terms, this type of work would be classed as short term and solution focused. Therefore, you would expect the duration to be weeks or months rather than years.
Following the initial consultation session, if you have chosen to proceed, a full and thorough assessment process will be carried out. A longer session is required for this as the information/history gathering part of the process is vital to build a bespoke treatment plan for the most effective outcome. For a couple, more than one of these longer sessions may be needed.