When I'll feel a strong and continuous emotional surge that will carry me to my demise. Clearly, being certain that death is a better alternative for me isn't enough, otherwise I'd be lying amongst the dead already. I've also had several situations in my life when I wanted to do something for a long time but didn't up until I was overwhelmed with strong feelings. They felt unpleasant but so true and right, like anger that is expressed, not suppressed. Distraction (video games) prevents these feelings to appear. Can't react when there's no trigger.
I also noticed that I have hard time talking myself into a ctb state while others can do that accidentally, without much effort. I explain it this way. My subconscious knows my triggers and somehow manipulates me into avoiding them. Other people don't know what hurts me, nor do they care if they do (if they think they can can get away with it, of course), so I figured I could gain suicide fuel by talking about suicide-related things with others.
I have a repetitive dream pattern (repetitive enough to notice it, but not too much) where I run away, usually from a group of people, and when I'm cornered or surrounded, I turn around and fight. I think I'm in a fleeing stage, distracting from unpleasant thoughts about my position in life. When my distractions become ineffective again, when I will be exposed and imbued with emotional force strong enough to go against my survival instinct. That is when I will strike back.
I'm not trying to tell you when it's your time though. Everything I wrote above is my subjective view on my own experience. Treat it as such.