Great question and good responses !!
I don't have a "primary lover" since I am a single female and I live alone, but I have 4 male lovers.
I had monogamous relationships with 3 of them that lasted 4 years, 3 years and 1 year.
I consider these three to be "long term" relationships since I have known them 10 years, 8 years, and 6 years.
I consider myself to be living a successful poly lifestyle because my lifestyle is a manifestation of my philosophy about relationships which has nothing to do with the "length" of a certain connection or configuration- in fact it's just the opposite!!
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.