To make real gains in therapy where you become a stronger and happier person, it's generally advised to do one session per week. A therapy course can range in length of time, but a basic course of therapy would be about 16 sessions. If your therapist sees you are getting better more quickly, they will recommend you either take a break or stop therapy entirely. But it's important to prepare yourself for a longer term commitment if you are serious about getting better.
After an initial assessment, your therapist will recommend how many sessions you should have and how often to do them.