Okay guys, he wants advice, not a judgement call of his lifestyle.
Now, going off the derech will be painful news to your parents no matter when you tell them. Young people often go off the derech, following a path that is attractive to them at that time.
You need to answer (to yourself) are you rejecting the PRACTICE of religion or do you truly not believe in G-D.
Will your parents will be visiting you in the near future? In which case they will see your “lack” of observance.
If not, you need to share with your parents, who love you very much, your doubts about the lifestyle you are choosing to leave, at this time. I strongly suggest that you do not shut doors behind you at this time. You need to tell your parents, but parents are usually quite intuitive when it comes to their children. I am sure they have “picked up” clues about your behavior and beliefs. It is up to you to share your ideas, ideals, doubts with them. DO NOT turn it into a shouting match, do not make it you vs. them. The conversation will be painful on both sides, but you need to start a dialogue with them as soon as possible.