This is a difficult question to answer as there is often a great deal of variability from person to person with respect to how quickly they experience a decrease in their OCD symptoms once engaging in psychological therapy.
Much of this variability relates to how severe your symptoms are, how faithful you are in completing the homework assignments, the skill of your therapist, your relationship with the therapist, and your insight with respect to the impact of symptoms and your motivation to get symptoms under control.
That said, generally speaking, most people with OCD can expect to have to complete between 12 to 20 therapy sessions to realize a clinically significant decrease in their OCD symptoms. More often than not, booster sessions are required to maintain gains made in therapy. Moreover, at the outset of therapy, it can be helpful to have sessions twice weekly, rather than once, to build momentum.
When undertaking cognitive-behavioral therapy, it is not unusual to initially experience greater anxiety than prior to engaging in therapy. This is natural and is a function of finally confronting many of the feared thoughts, objects or behaviors that were previously avoided. Many people report a significant drop in symptoms early in therapy, with slower and steadier gains as therapy progresses. However, this is certainly not always the case and there are many ways to progress “normally” through therapy.
If you're experiencing especially severe symptoms, including engaging in rituals most of day or being unable to leave the house owing to obsessions or compulsions, a prolonged course of treatment (sometimes many months) in an in-patient intensive treatment program may be advised. In these cases, you would receive a massive “dose” of psychotherapy through the completion of daily exposure and response prevention therapy for months at a time.
The good news is that you can greatly influence how long it takes you to benefit from psychotherapy. The more consistent you are in attending therapy sessions and the more homework assignments you complete on your own, the faster you will see results.