According to Denise Bartell, PhD, a psychologist at the University of Wisconsin, Green Bay, the "in-love" stage of a love affair normally lasts six to 18 months, and can last as long as three years. However, it eventually fades. What ends the in-love stage of your relationship is that neither you nor your spouse can find anything wrong with the other person. At some point, problems arise between you that cannot be resolved by just talking it over. For example, your spouse may feel ignored or unappreciated and want to have an affair to feel loved again. Or perhaps he/she feels like you don't understand them or their needs. In her book, The Marriage-Go-Round, Bartell says that most marriages experience an affair at some point during their lifespan. Marriages where both parties work on their relationships and try not to hurt each other's feelings during this time usually end up being stronger than others. But even these marriages will see their infidelity rise and fall over time, because people are human and make mistakes. The only way to really know how long this marriage affair has been going on for is if one of you comes clean about it. If you're lucky enough to find out before you walk down the aisle, you have time to think things through and decide what you want your future marriage to look like.