I heard a lot of affairs will not last longer than one year, and most affairs will not end up with marriage. Is it true?
Most don't, however:
I've seen wildly conflicting data on this, and it's debated pretty thoroughly on this forum over the years. No one's really sure where the "most affairs last six months" claim comes from. I think perhaps one-night stands are factored into that, so that probably pulls the averages downward.
I try to focus on how much emotional, financial and even medical carnage can be wrought each month the affair continues, and how much the emotional bond deepens, when encouraging betrayed spouses to fight like h*ll for their marriages.