wow, what a great question!
Nerdist and I met as a result of my girlfriend and I becoming non-monogamous 12 years ago. We thought we would find another girlfriend, but I found a man and she had a hard time with being poly... strangely enough not that he is a man. Even though we were die hard lesbians at the time.
We have been married for about 10 years and have always been polyamorous. Mono has been the longest at 16 months though. Others have come and gone. I see them as a means to be ready for Mono... we were so ready for someone who wanted to commit to us for the long haul. Even though he is my lover, it has really worked better that he has become part of our family and not just a lover of mine. I think that should sustain us longer than any other lovers that have come and gone. We shall see