I, after some admittedly short research found averages for monogamous relationships (including marriage) anywhere from 18 months to around 4 years. Which surprised the heck out of me. I’ve been Polyamorous for the last 24 years and the shortest relationship I’ve had in that time is 6 years. Most of my poly friends are also in quite long term relationships but that may just be a function of the friends I make.
This is also an entirely anecdotal response to your experiences as well.