Fifteen years.
Day to day, I don't think about it at all. It's a scar, and always will be, but no more.
My dh had an affair with someone I had called a friend. We had lots of mutual friends, and our children were friends. In the immediate aftermath it was very difficult to deal with all the facets of betrayal. He cut contact immediately, and we still have the principle that I can look at his emails, phone, whatever, and he doesn't password protect things from me. He gave up the right to that privacy, and to be fair, he gave it up willingly.
It took about 6 months for the constant aching pain in my chest to go, and about a year before it wasn't the first thing I thought about when I woke up.
After about 5 years I felt reasonably confident. Other things in our lives had changed, and that helped too. I started working full time again (had been a SAHM), dh travelled less for work ( which helped in lots of ways), and we were enjoying being married again.
Now, I would say my main source of ongoing grief is the lost friendships. I lost contact with the woman I had thought was my friend, and I still miss her, or who I thought she was. Our many mutual friends mostly didn't choose between us; they see us and them, just not together.
But now our children are grown up, I miss her kids' weddings, big events in their lives that I hear about third hand. I regret that my children lost those friendships too. I also hate that our mutual friends know so much of what happened. I would prefer that our problems had been private, but that wasn't possible.
A few times I have been at an event where the OW was there too. We usually say hello and move on. Once or twice I have asked how she is, how the children are doing. I know we will never have any more than that, and I miss her.
I didn't tell my parents about the affair. At the time that was hard, but now I'm glad. I wouldn't want to feel that they were thinking about it whenever we visit.
I'm glad I stayed. We have a good marriage, in many ways better than it was, but there will always be regrets and niggles.