Exposure and Response Prevention (ERP), like most Cognitive-behavioral treatments, is a time-limited therapy. The objectives of the treatment are to reduce your symptoms and get your life back on track, but also for you to gain the knowledge and learn the skills you need to maintain your gains without a therapist.
In treatment sessions you learn and practice new techniques and skills with the therapist, and then between sessions you continue to practice what you have learned on your own. How quickly you feel better depends a lot on how much time you are able to devote to this practice between sessions.
Another factor is how intensive you are getting the therapy. Research has shown that symptom reduction is most rapid when sessions occur from two to five times per week for up to two hours per session. But again, the more you are invested in practicing between sessions, the better and quicker your results.