I googled "stability of cohabiting couples". This led me to Wikipedia [1]. From there I hit the 2002 CDC report "Cohabitation, Marriage, Divorce, and Remarriage in the United States" [2]. That provides the following statistic:

"The probability of a first marriage ending in separation or divorce within 5 years is 20 percent, but the probability of a premarital cohabitation breaking up within 5 years is 49 percent. After 10 years, the probability of a first marriage ending is 33 percent, compared with 62 percent for cohabitations."

I wondered if that had changed since 2002, so I hit Google Scholar and searched for articles after 2010. A 2020 article discussed cohabitation broadly. I scrolled down and found a 2018 article [3]. (There's an ungated PDF elsewhere if you need it.) They look at 8 countries including the US. Their findings: "cohabiting couples who do not subsequently transition to marriage... consistently have the highest predicted probabilities of separation within 5 years." Some of the gaps between cohabiting and married couples disappear if you control for (e.g.) education and other demographics.

[1] https://en.wikipedia.org/wiki/Cohabitation#Likelihood_of_spl... [2] https://www.cdc.gov/nchs/pressroom/02news/div_mar_cohab.htm [3] https://read.dukeupress.edu/demography/article/55/4/1389/167...