Probably after my dad dies (but my feelings are torn on this because I truly hope that he lives a long, long time). I hate the idea of leaving him all alone in this awful world to fend for himself. If I don't CTB at that time, then definitely when I can't take care of myself without someone else's assistance. Or when I receive a terminal diagnosis.
On the other hand, I don't know. I fear death so much but life is equally terrible. Consciousness itself is terrible. I might CTB earlier if I have to.