Several things affect how long, or how many appointments clients require. You have probably heard of some individuals who have been going to therapy regularly for several years. This is a fairly common timeline with an approach such as psychoanalysis. However, most therapists use briefer forms of therapy.   This is due in part, to research data on therapy cost-effectiveness, and the fact that insurance companies limit the number of sessions they pay for.  However, it is clear based on a large number of quality research studies that the rate of improvement with most forms of counseling often starts to decrease after about five or six appointments.
A second factor that can determine the duration of therapy is the severity of your condition and the number of problems you face. Relatedly, whether or not a particular diagnosis tends to be short term versus chronic is strongly associated with how long therapy takes.  For example, depression related to bereavement tends to require shorter therapy than true bipolar disorder, which is often a life-long condition. Also, some problems such as addictions (alcohol, pathological gambling), tend to involve many relapses along one's way toward recovery.
Situational changes during the course of therapy can influence the duration of that therapy.  For example, a client secures an exciting, high paying job after being fired can improve his or her depression significantly.
A factor that leads to longer-than-needed therapy has to do with the therapists’ use of inefficient approaches to therapy.  A great deal of research demonstrates that the vast majority of therapists are not using the best, empirically validated treatments with their clients.  Relatedly, most often, when therapists claim to be using a particular type of therapy, they are not actually implementing it adequately with their patients. Our review of over 1000 social security disability medical and therapy case files showed that therapists commonly claim to empirically-supported treatments such as "Cognitive-Behavioral Therapy (CBT)".  However, their case notes nearly always read like a diary that documents the life events experienced by their client since the last appointment. Rarely is anything ever mentioned about use of specific CBT procedures, their patients’ response to CBT methods, or whether symptoms were improving since the last appointment, or whether clients completed a CBT "homework" assignment. Therefore, the simplest conclusion is that CBT is not actually occurring in the sessions, which is almost certainly inefficient and unfortunately, tends to needlessly extend the duration of therapy.