It happened over 6 years for me. I woke up one day and realized that all the "love" I thought I felt for my x was gone. I was just beaten down by the chaos to the point of not caring any more.
I wasn't being nurtured, I was being used.
I had no energy to do the simplest thing any more.
There was never a "good" day with him any more.
Those things added up and I realized I needed help and I needed peace.