It's different for everyone, but here's what most people find.
Frequency of appointments
While there is some conflicting research on the relationship between session frequency and psychotherapy outcome, research largely shows that for all diagnoses, improvement and recovery are associated with higher frequency (weekly or fortnightly) appointments during the first three months of therapy. This is especially the case for individuals with higher symptom severity and or greater functional impairment.
In other words, at least for the first 3-months of therapy, go often!
Length of treatment
Research has generally found a positive relationship between treatment length and clinical outcomes such that more individuals will show significant change or recovery with increasing treatment length. There are a growing number of specific psychological treatments of moderate duration (e.g., 12 to 16 weekly sessions) that have been scientifically shown to result in clinically significant improvements.
However, this will vary greatly depending on your experience with therapy, your therapeutic alliance with your therapist, the complexity of your mental health needs, how able you are to action your therapy tasks, and many other factors. You and your therapist will make this plan together.
Reviews of the efficacy and effectiveness of psychotherapy demonstrate that therapy is highly efficient for a large minority of clients, perhaps 30% of whom attain a lasting benefit after only three sessions.” And, then an even larger number of clients respond to a longer set of sessions, “...It appears 50% of patients respond by the 8th session and 75% are predicted to need at least 14 sessions to experience this degree of relief.”