There's lots of ways to arrange open model relationships.
And in the polyamorous segment -- as many ways to "do" poly as there are people.
All relationships come with a clock attached. Even "til death do us part" is an ending.
Once upon a time I was a hinge type person ever before I knew the terminology. I married then BF1 and we have clocked 19 years together. BF2? From active friendship to romance back to active friendship to fading from my life? 6 years. He's still around to ping by email if I ping him. Mostly I don't -- he's not a pen pal type. So is it 6 years or also 19 years? When does one stop/start counting? What about natural pauses in between because of school, work or whatever? Does it matter? I don't think it is worthwhile to stress out about it.
It will last as long as it does. And even when one thing ends (romance), something else may continue (friendship). I'm fortunate to be on good terms with my exes even if we're not especially tight when everyone's spread so far across the country.
In friends -- I don't see that the monoships or polyships last longer than the other. Largely it depends on the people in them. A church friend just lost her spouse -- they were married (and monogamous) for more than 50 something years. Meanwhile, the oldest polyship I know? Friends with my mom. By my estimate it's like 44 years.
What is it you need reassuring on?