I was just reading "Renewal Coaching" by Douglas Reeves. He explains studies where researchers can tell within 15 minutes if a couple will still be together 15 years from now.
It has to do with respect for each other. If a partner shows disrespect for the other, holding contempt for the other's views or ideas or beliefs, that is a fairly sure sign they won't last. If they criticize, or cause the other to become defensive, those are all death knells.
So often it's not about length of time - it's about the amount of patience and respect and "calm discussion" ability each person has.
It's a great book - here's a straight, non-affiliate link for more info.
http://www.amazon.com/Renewal-Coaching-Sustainable-Individuals-Organizations/dp/0470414960
