How do you know when it's time to leave a relationship?

As soon as possible. Yesterday. Immediately. Why would you stay in abusive relationship another minute?

Immediately most of the time. If you talk to the abuser and they are willing to get help, it's up to you if you want to stay and try again, but the person has to be willing to get help and do it right away.

When you feel "dead" inside. The sooner the better though but sometimes people wait to hit rock bottom before realizing they have to leave the relationship.

You should leave the minute you are abused. My husband and I were having a verbal fight and he raised his fist and punched his hand towards my face. This really scared me so, I immediately moved out and we are separated. The moment he did that to me. I thought maybe next time it would be my face. Being that, I had experienced a physical abusive relationship in the past. I instantly flashed back to what happened. This is why left.

If you've tried to understand why he might be out of out control and impossible; (He's under a lot of stress, We all have our moments etc...) and you realize that your acceptance and understanding have been mistaken for permission to continue to disrespect and mistreat you. When he tells you that his anger and mistreatment are your fault. When you start avoiding your friends and relatives because you're so ashamed of yourself. When you find yourself making excuses for his behavior, but he won't forgive or understand you.

IMMEDIATELY - When we notice the first "Red Flag". Why would we want to take the abuse? Why would we want to waste any of our time being disrespected and treated like we aren't even human?

As soon as you noticed it was abusive, you should no longer be in that relationship. In other Words, now.

When you're ready, trust your gut. This may sound insane, because the obvious answer is to get out as soon as possible; but if you're not ready to leave then you'll probably fall back into the abuser's grasp. Its happened to me. Your best bet is to start quietly separating yourself from them- disconnect your emotional state from theirs. Put away things they've given you, do anything that allows you to disassociate yourself from them so it is easier for you to leave. When you're out of the relationship, burn those things you hid. It makes things more final, and you'll be less likely to fall back on those things, or worse, your abuser for comfort.

I'd say, when you start asking questions like "how do you know when to leave an abusive relationship".