I don't think it was ever specifically mentioned. I don't remember which episode it was in (one of the Christmas ones maybe) where she mentioned seeing her father kissing the neighbor lady and I think she mentioned she was nine.
In Lucky Leon during their date Lois says:
then my senior year in high school I had a huge fight with my dad and I moved out.
It doesn't mention specifically that her parents were living together at the time. It's possible that Lois moved in with her dad due to some altercation with drunk Ellen.