My P told me that if a RS lasts a long time both are pretty healthy or they both are really sick.
Sounds quite plausible. My uBPDxgf has had 4 relationships, including ours. (Possibly hundreds of guys, but only 4 'relationships'.) The first two were both reasonable, average guys (according to her brother). Both lasted less than two years, and both guys were left holding the baby. Literally.
Her third relationship lasted
nine years, and that guy is one of the most narcissistic, aggressive, moody people I've ever had the displeasure of knowing.
Her brother told me that of the four, I was the only one to make a genuine effort to understand her and help her deal with her issues. (For example, I encouraged her to reconnect with her FoO and children, from whom she'd been estranged for some years, and I regularly drove her the 500km return trip to visit them.)
How long did I last? You guessed it! Just on two years.
(Don't mean to imply that you're sick, arn. You're probably the exception that proves the rule!

)