I asked this question in a different way once...the consensus seemed to be that "successful" does not necessarily equate with "long-lived".
My primary and I have been married 20 years this summer. (...wow...) We were monogamous, I never wanted to be anything else. Had poly friends who gave me a very bad impression of poly.
My secondaries have been friends for years. Three years? Four? They gradually became closer and closer. A couple of years ago we started hanging out together as a family every weekend. About a year ago we made the decision the be a quad. I think that means we've been romantically involved for a year, but there was a lot of lead-up before we committed. (For example, just how romantically involved is strip poker? Does it count if Sunday groped me when he was drunk? Does it count if Sunday and I were "married" on weekends when Asha and Easy were working, and proudly showed off our combined brood of children? Does it count if I'm absolutely certain that Asha and Easy were romantically but not sexually involved with each other before we decided to become a quad? Hmmm...and I specifically remember proposing marriage to Asha two years ago...)
I know what you're asking, and I don't think there's any easy answer to it. (I wish there were.) Poly relationships break up, just like mono relationships do, and there's no way to know if you're going to work in either format. All you can do is give the relationship your best effort and hope.
Are we successful? I don't know. We're always working on some kind of issue.