I drew the line at the precise moment when I realized that my life was actually in danger, when I saw that the violence was only getting progressively worse over time. I decided that my children would be better off with me alive rather than dead, even though I wished I were dead because the depression became so bad.
Both situations are bad, physical and psychological abuse. The physical is just more dangerous physically in that it could cause death and therefore requires a more expeditious resolution, i.e., separating the abuser from the abused, to prevent that, in my opinion, whereas there is more latitude in the psychological abuse situation, absent the life and death urgency of the physical abuse situation, to give counseling a try, if the parties are agreeable to that, in my opinion. That would be my advice if I were asked.
Everyone has to draw their own lines, no one can or should do it for them. People can seek advice and counseling, but they still ultimately have to decide for themselves what they are going to do, what the best course of action is for them.
And I agree, no one has to wait around for fists to start flying before they say enough is enough, everyone has their own breaking point and they know it when they reach it. And generally speaking, people know pretty early on in a relationship if physical abuse is a likelihood or not, so if someone does decide to wait, they won't be waiting too long.