There is no set amount of time as everyone is unique and brings different concerns to therapy and has different goals in mind. Some therapies are brief (4-12 sessions) and some are longer term (more than 12 sessions.) However, you should feel that therapy is helping you. Even if your goals are longer term or you are addressing many concerns, you should feel that you are making progress. If you don't, you should talk to your therapist about your goals and what progress you are making.