You'll know when its time to let go. Its like something in your head snaps and you just know you can't stay with this person anymore. Its hard to explain but I don't have any doubts about my decision. I am sad. I am scared. But I also know I am doing the right thing.
When I first started DBing I constantly questioned, how would I know when to end it. I was afraid I would end it and regret it. I started leaning towards ending it the last 6 months (before OW#2 even showed up). I started seeing more negative than positive and being around my H was starting to irritate me. I no longer felt the drive to fix anything and secretly hoped he would just leave. I knew then that ending it was in my best interest. It took OW#2 showing up at my house for me to call a lawyer. I was getting there and would have reached that point eventually. OW#2 just hurried up the process.