I'll say this, purely from a reasoning standpoint and not from a research stand point. With my wife, we dated for a year, cohabitated for a year, and were engaged for a year before we were married. This made sense to me because you get to experience all the holidays at every point in your life, and it let's you see each other how you exist throughout different points in the year. If you need the AC cranked in the summer, and your parent can't stand that, or if you want to bake all winter and stay inside while your partner wants to go out all the time, you aren't able to discover that without cohabitating for at least 1 year.

i would consider it a well-known fact that there is a link between cohabitation before marriage and higher rate of divorce.

granted, divorce rates are rising anyway. but to be totally honest, if OP considers this boomer talk, it just speaks to lack of insight and life experience (which you will get when you move in with someone, to be fair). also, looking for an exact number to reach some kind of threshold just seems like a cry for validation. you certainly don't need to gain approval from people on the internet to make a decision (myself included). you won't need to know a number when you're ready, because you'll know the time is right.

regardless of what i said, i hope you find further research on the matter (try using pubmed or national institute of mental health resources) and i hope you find happiness if you're taking that next step in life.