I think people are wired polyamorous or not. To me that is the ability or desire to love more than one romantic partner at a time. Monoamorous is the ability or desire to love one at a time. What shape their current relationship status is? That's another thing. Single, dating, partnered with 1, 2, 3... all that stuff is relationship configuration to me. A person doesn't stop being polyamorous just because they are single.
I don't know that there is any statistics for length of polyships of various configurations. (V's, triads, quads, etc.) It's all so DIY and up to the cast of characters inside the polyship.
I believe all relationship come with a clock attached -- even "til death do us part" is an ending -- be in in a 2 person thing, a 3 person, a 4, etc. That's just reality. Sometimes people break up because of school, work, changes in how they feel like falling out of love, etc. Polyships are no different than monoships in that sense. The relationship will last as long as the people wish to stay in relationship -- and that includes the relationship changing shape from friendship to romance to friendship again to... whatever it is the people want.
Maybe thinking that if I just prepare well enough, I can avoid some pitfalls is Pollyanna thinking.
Maybe this could help?
Avoid the Pitfalls and Reap the Rewards of Polyamorous Relationships
Or just websites in general?