I'm having cbt and also celexa (citalopram) for my pure-ocd, anxiety and depressive mood. How long should this last to prevent relapse? Scared of taking it forever

No definitive answ. Each case is unique while sharing symptoms with others. You may need some chemical assist all your life and maybe not. You can certainly learn to manage the symptoms but you have a pretty complex pattern there and I wouldn't expect to have a short term fix that will last. You and your therapist and doctor need to discuss prognosis and their impression of how long and what you can expect.