Twice in the last week I've heard of 2 different marriages where this happened.
Someone I work with told me a very good friend of hers got divorced after his wife left him and moved across the country to live with the man she met on the internet. Yes, she left behind a child with her husband. A year and a half later, she moved back to the area and wanted to know if they could work it out. He said NO WAY! It's too late.
Friend said BS is happy now and has a new S.O. that he met through a support group.
Saw my BIL (from ExH's side of family) this weekend. He stated that his 1st wife wanted a divorce after 2 1/2 years. She wanted him to be management, make more $, etc. They were her goals, not his at the time. He wanted counseling, she didn't want to go. They amicably divorced and remained friends. They would get together occasionally and catch up. After one of these dinners, I'm thinking it was about 2 years later, she said "I made the worst mistake of my life". He said "Sorry, but it's too late now. There's no turning back."
Ironically enough, he (who's happily married to my SIL) is making a lot more money now and is "management". He said his xwife will spend her life looking for others to make her happy and will never find happiness.
Neither of these people knew about MB. It appears sitch #1 was Plan B'd and it was right under the 2 year mark Steve Harley talks about.
Sitch #2 appears to me a Plan A followed by a 180 that changed her mind.
Again, in both cases the spouse left behind decided it was too late to try again.
I really, really get the feeling that in sitches that will turn around, it seems to happen close to the 2 year mark AND when the WS feels the BS slipping away and moving on.