Only you can decide when you've had a long enough time to show him your positive changes, and when it is time to kick him out into the cold harsh reality of his choices.
DBing is NOT about being a doormat - when a man is carrying on an ongoing affair right in front of you, and tells you he's not willing to give it up - it may be time to splash some cold water on his fantasy.
BTW - does this woman's husband know about the affair?
I would suggest you start planning for your future. See an attorney to get an idea of what your financial position would be. Squirrel away some emergency cash, or if you can't, at least stock the pantry with lots of canned goods in case he goes off the deep end and you need to squeak by until a support order is in place. Start going out with friends and building your own social life. Think about what you might do for a living if you were single.
I'm NOT saying it is hopeless or over - quite the opposite, I suspect when he has to face reality he's likely to want to come home. He's just caught up in the intoxicating infatuation phase. And we all wish he would come to his senses before the kids have to hear anything about it. But I think unfortunately this is not going to be that simple, he sounds like a guy who has to be woken up out of fantasyland. (And I have to ask you - was he ALWAYS this monumentally selfish? He would be "bitter" if not allowed to continue his affair? Really????)
So - now is the time to GAL, create the life for YOU that YOU want, take up new things, make new friends. Get your financial ducks in a row just in case. Get a makeover, cut your hair, lose weight if you need to. Create a little mystery - don't date, but it's ok to let him think you MIGHT be going places where you MIGHT meet other men when he's out of town. (Stupid but true, the "feelings" for you often kick back in when they realize they could lose you to another man).
Right now he has all the power as you are waiting for him to choose. It may be time to take that power away.
