It didn't effect our relationship. I felt guilty about it. I told a married friend about it and he said basically what I told you. That made me feel better.
Becoming parents did change our relationship dynamic a lot. We ended up adopting because of fertility issues. We went from no kids to having a toddler with ADHD who hated sleeping and couldn't speak English overnight. It was a big adjustment and we pretty much didn't have sex for the first couple of years.
We still don't have sex as much as we used to. We'll get there eventually I hope but things are fine. We talk about it. I think the long stretch with no sex kind of lead to me mostly fantasizing about my wife.
So far as how long it lasted with the cheating fantasies it was really intense for the better part of a year. I still have them occasionally but it only happens every once in awhile. When they do I just get myself off. Once I finish I stop thinking about it.