When it is the only answer that makes sense for the individuals involved.
There are no guidelines, only feelings.
If you are miserable, you are not doing yourself or your kids any favors by staying.
I stayed with an abusive husband for 20 years. I raised my 6 kids and shielded them as much as I could, but they still were exposed to verbal abuse. When I finally said I was leaving my these grown kids showed up with boxes and 4 trucks!! They had me out of there in less than 3 hours! When I got to my new place the daughters in law were there hanging pictures and putting in the cupboards! All they wanted to know was why I waited so long! I think when you can't stand anymore and can't think of anymore reasons to stay...you just know when its right for you.