This differs from person to person and depends on the nature of what you are bringing to therapy. Often the longer the issue has been ongoing, the longer the time needed to work through it.
Usually after the initial session I contract with the client for six sessions and thereafter, if the client so chooses, continue on an open-ended basis.
When working open-ended I ask for a minimum of two weeks' notice to finish therapy so that we can work towards a healthy, managed ending.