For me the time to leave was the day I finally quit second guessing myself...I think deep down I knew the truth, but I wasn't ready. When you are ready you will know and if you are not ready keep working on yourself and the time will come (or it won't, but you will at least be getting yourself healthier).
Something I found really helpful when I finally made the decision to separate was..."I can always change my mind, but I need to make decisions based on what is true RIGHT NOW." IF things get better then you can make decisions based on the new truths that come your way.
Sometimes it is hard to see people and situations for what they are without all the what ifs and the what has beens, but the truth of the moment is the only truth and it helps me to focus on that when I have to make tough choices.
I wish you the best!