If you want to stay friends, never. This is a bad idea if you value the friendship, even if she would tell you it's okey. Marriage is a big commitment and if they were married, they have strong feelings for each other that never go away, even if those feelings may no longer be passion and romantic love. She may not realize how much it would hurt her until it happens.

If you are in love with him and see a potential future with him that you value more than your friendship, there are still a number of potential problems with this. Depending on how close you are to your friend, she might have shared personal information about you with him and you might now personal information about him, including his "flaws" and details of their arguments. This will not allow for the two of you to get to know each other in a normal way. Your other friends will see you as a potential threat to their relationships and might consider you not to be loyal. If your friend and you hang out with the same people, socializing might become awkward for everyone as the two of you start dating.

Personally I would never even consider dating a friend's ex! Just a huge no-no, in my opinion. If you are truly in love, even then I'd really think very long and hard before jumping into a relationship with him. Even if your friend says she doesn't care - what is she supposed to say, you know?

I think at this point you really should think about your friendship. How much does it mean to you? If it's truly valuable, rethink your plans. Hands off.

Or wait 20+ years, or more. And then maybe a little longer for good measure.

I think that's a really touchy subject. I don't think there is an ideal time frame to start dating a friend's ex husband but if you do pursue this relationship you should be fully prepared to lose that friend.