I found out after 9 years - he'd been having an affair for three years at that point. I wasn't "glad to find out" - and I was even less "glad" when during my research I actually found out that he'd had a previous affair, starting within months of us getting married. I also found out that I was, as they say, the last person to find out. We lived in a small town and it was obvious to me that many, many people must have known while I was oblivious to his activities. I even had my 2nd child during that period, so you could say that I was devastated.
It's not so much the sex, etc, which destroys you . It's knowing that he has been lying and lying and lying to you. He'd even told me that his friend had died and he had to travel to his funeral - turns out he had a "weekend away" with his girlfriend . And knowing that every Christmas, every birthday, every special day together....he'd been thinking about her instead.
I certainly did get a divorce - best thing I ever did. At least then I could make a fresh start. The idea of staying, getting counselling, etc, would not be for me. I'd have always lived with the knowledge that I was living with someone who thought nothing of betraying me, and telling me untold layers of lies to maintain his " having cake and eating it" lifestyle.