My doctor said 3-6 months too, but that was just an estimate. I think it depends on the person--the goal is to change the chemical environment in your brain so you aren't as anxious or depressed. It's scary at first, I'm starting on one right now too. My mom has had a lot of success with this kind of medicine and she says whatever the side effects, it's worth it--I'm gonna stick it out.
I hope this helps a little.