For me, enough was enough in two stages: the first was after my therapist told me that she thought my being involved with XABF (who was at the time not X) was just holding me back from meeting someone who could be a real partner to me. I got pretty angry at her, telling her that she wasn't allowed to make judgments like that. But even then, I suspected my anger really came from a place of knowing she was right but needing to continue to deny it.
Stage 2 was instantaneous. After several nights of falling asleep drunk on the couch, one night around 1:30 am, he came crashing into my bedroom, screaming obscenities at me at the top of his lungs. My first coherent thought was that my therapist was right, and my second was that she was right to say it to me. He was out of my condo and out my life within the week.
So I guess I am saying that first I had to stop denying that my life was the way it was, and stop all the magical thinking that there was something I could do to turn him around. That took months. It wouldn't surprise me if it took other people years -- nor would it surprise me if it took other people days. My therapist's words put my focus back on me and what I wanted -- and then I really just needed to get out of my own way. Once I got there, recognizing the moment when 'enough was enough' was super-easy.
Just my experience. Take what you like and leave the rest, as they say. I hope you come back often and keep us posted on how you are doing.