My husband and I were monogamous for 15 years before deciding to actively live as poly. We are now separated, but we were poly together for three years (and are still friends). I was with one partner for just over three years, and my four current relationships range from 14 months to 2 months.
That said, I wonder if time is always the best indicator of a successful relationship? Isn't success also measured by how you feel about, relate to, and communicate with someone, regardless of how long you spend as their partner?