With 1st XP (I was 19-22), it should have been when:
-he screamed profanities at me....because I'd shaved my legs.
-when he would give me the silent treatment (or worse) if I'd somehow done something wrong that he didn't like (NB he'd change the goalposts for what was/wasn't acceptable, so I needed to be on the ball for this).
-when he made me feel like worthless/accused me of being unfaithful (something I've never done) for having male friends. OR indeed any friends that he didn't approve of.
-when he'd cut me down to size when I did well at uni/my job/whatever....but equally if I didn't try hard, I'd be stupid worthless etc.
-when he tried to stop me seeing my family.
-when he told me that no one would love me like he did. Because I'm fat, worthless, stupid etc.
-when he'd insult me in front of his friends (they'd often step in and say something....some of them).
I developed a stammer because of his abuse. I don't know what it was that made the scales fall from my eyes, but one day I woke up and had had enough. I think I figured that whatever I did wouldn't be right. So I got the hell out of there. I think he wanted someone who was academically good, outgoing, confident etc (I was like that before we dated....so was his ex before me)....but because he wasn't successful/confident etc (always someone else's fault, not his), he'd wear us down until he got us where he wanted us....but then he'd berate us for not being like we were before he dated us.
With XH, I should have left when:
-he started going AWOL due to drinking (and I found out later drugs).
-when the verbal abuse first started.
-when the lies first started.
-when I got knocked about (this was just the one time)
-when I started feeling scared about going to sleep/going to work etc and be scared about what would set him off this time around.
Straw that almost broke the camels back was him disappearing off on one Xmas Eve (to get drunk/high), me trying to look for him and then me finding out from neighbours (who were helping to look for him) that he'd been drinking (and taking God knows what) and then driving to pick up the kids. I stopped him having the kids unsupervised straight away and stupidly even then(!) said that if he could get clean/sober, we had a chance. What finally did it was me realising he was continuing to drink/do drugs/lie about it because he DIDN'T WANT to quit doing that shit.....oh and the fact he'd been fucking a "friend" behind my back (whilst promising he was clean/sober and wanting to work on things). They both did me a favour but I wish so much that I called time on it years before.
The above stuff is now why I normally say to people to get ducks in row/run like fuck if partners have alcohol/drug problems.....or are abusive.