I dated within a few months of my ex leaving. BUT I had been DBing my marriage for years, had reconciled once before, my ex was a three time offender, and I was completely sure that I would never take hm back under ANY circumstances. I was done and really healed at that time - I had peace in my heart that I had done everything possible for my marriage, and peace that I did not want him back. Just because it took another year to complete the legal process of a divorce, I was more than ready to date and I did.
Some people's divorce drag out for a long time for reasons other than attachment. And some people went through the necessary emotional process before their ex left. I'm not saying it's common, necessarily - and it's a reason why you have to assess whether someone is truly done with their marriage or not.
Ask Andrew how he has made out dating married people.