How/when should I tell my parents what I believe?

My parents are very conservative christians and very strict people. I am an atheist and a very relaxed person; i've never really believed in religion. Throughout my life, my parents have done everything indoctrinate me with christianity including sending me to christian school, youth group, and forcing me to participate in religious ceremonies. I've never actually told them I wasn't christian, but I also never put any effort to look "christian". I smoke cigars, occasionally drink, and date "non christians"( in my parents eyes, thats hell-worthy). I just try to enjoy life, but I have to go behind my parents back to do it. I know if I told them what I believe( even avoiding more problematic philosophical areas, they would kick me out and wouldn't support me through college- I'd be on my own. I could continue to lie to my parents through college, but its hard work and isn't the way I want to live my life. I feel like they should know where I stand. How and when should I do this?