, excuse me for being a little giddy but this is something I know all to well and can give you very positive information about. BTW my wife also left me for an exit affair with a loser friend that chased her all through our marriage (frekkin b*stard) but I got the upper hand in the end knowing exactly how this was going to go down.
The short answer you're looking for is about 2-4 yrs, maybe less if you can approach him with some gentle interrigative tactics and let him know that you knew all along and how you're happy he's with the love of his life and a far better woman. This is more of that "act as if" philosophy at work, but at the same time by disqualifying yourself and chocking it up as "not meant to be..... whatever it's no big deal". You take away his defences by reassuring him you don't want him (in a nice way) and he'll wonder why.
When I confronted my wife I got her to admit it twice, but I could only pull about 50% of the truth before she started to shut down and I had to switch to a happier lighter subject. I got my answer andlet her know I was happy for her and it's no big deal she's with the OM. Secretely, I was laughing at their pathetic relationship because I know statistically they have a greater chance of both being hit by lightning.
The reason these relationships fail or end badly is because of #1 they can't ever truely tust each other. How can she start a family with him when he's abandoned you and his babies, but also they are both cheaters and liars. #2 They said "I love you" long before they every deserved to hear those words and by qualifying her as the greatest thing ever before he's had a chance to know her... he kind of shot ignited th fuse on a stick of dynamite there. Honestly, 10 months isn't even long enough to know someone..... they probably haven't even had a chance to have a serious fight and grow closer yet. No, they're still in the honeymoon phase and will be for about another year or so. #3 Their relationship is loosely based on sex and fleeing from all responsibilities. You probably already know this one but it's important to point out that.... duh, she's 18 and willing to learn but an emotional idiot that knows nothing about life yet. She's just getting her kicks while she can. #4 and my possible favorite.... Ok, she's 18 and he's how old???? What makes you think their relationship will last when she grows full of herself and sees him as a creepy old man and her as a 18 year old who can get with anyone she pleass.
I also have to point out the importance of rejecting him while he is rejecting you. He will do anything he can to fight for her because he knows she is more likely to leave him than you are. The percieved threat with you is lower because he thinks you'll one day take him back or give him another chance do to te kids. Once you reject him he will find interest in you, but once you reassure him he'll drop you like a sack of garbage to chase after her. It's going to take a lot of effort but you really need to take a strong stance to avoid him, ignore him, and make it clear you've forgotten him. You'll have another chance to start over in a few years when this OW doesn't work out and he's been used and tossed aside many times.... you may not want to see him now but you never know what will be in 5-10yrs.
And don't worry about thet b*tch flaunting her relationship in front of you. She is just jealous because you have more of a history and did everything with him first. You wanna mess with her head a little.... act like you could care less, agree with your husband (and ignore him/don't chase him), and then treat her with kindness.... all the while knowing her days are numbered and there's nothing she can do to save their relationship once the honeymoon is over and the real issues of trust, comfort, boundaries, and deeper commitment come into play. Like it or not she may have won the battle but the war is not over.