My usual response to that question is "when he threatened to kill me and our children and commit suicide."
But the truth is, I knew it was time to leave much earlier than that. I think one turning point was when I turned down a promotion at work because it included travel, and I didn't feel comfortable leaving our children alone with their father overnight. That's when I made the decision to start working toward planning to leave. He forced my hand by threatening our lives, so it happened quicker than I had thought.