I think the "best" time is to do it when you have a proper plan and have done a deep research about your method, have left all your goodbye letters/videos, bank account and credit card passwords so as to terminate your accounts more easily, etc.
Once doing that, you'll realize when the best time to leave is. It could be when you just can't cope with life and can't even leave your bed at all. In other words, it could be when you stop functioning as a "normal" human being.
I was like that some months ago. I was gonna ctb on Jan, 21st and it felt "the best time to leave" but I regretted it and decided to give life one more (and the last) chance.