In my case, H left me to pursue the affair. I didn't know that at the time. I think he couldn't handle the guilt and knew that I would soon find out if he stayed. It also allayed his fears that he was having an affair. You can't call it an affair if you've left someone right? ...
I would guess I really knew in my heart 6 months after he left but refused to believe it. I 'waited for solid proof' and I didn't get that till he moved in with her. Even then I had to assume because I only could see via face book and even that was tenuous - there was no announcement.
I of course wanted to believe the best in my h, that he would never cheat. I buried my head and the relationship continued to develop without any consequences. I have to add here that his family said they suspected that he had someone else but I wouldn't believe them. They never actually came out and told me when they actualyy knew. In any case, I was so loyal to my h and wouldn't believe he could do that. To anyone out there I would say take those blinkers off. There is no good in ignoring something that is happening. If I had known there was something going on I would have changed my tactics completely.
I have to say that when I had my suspicions about him moving in with her I still did not truly believe it. I called a DB coach and she just said to me, if you suspect it is probably true and then advocated and helped me compose an email telling him very calmly that I knew he was having an affair. Even now he still has not admitted it to me, even in response to my email. We meet and it is not discussed. I had no one to expose to as we do not have any mutual friends together any longer as so much time had passed - a year.
I know he feels guilt, I can see that, but has faced no repercussions to make him admit what he has done. I buried my head and did not 'expose' the affair - even if just to him and it hasn't worked for me so far.