From a reader:

Dear Jonah,
I have (unfortunately) some familiarity with common law marriage. While I can not speak to every state’s unique laws, as a general principle, the specified period of cohabitation is not the only thing that can establish a common law marriage. Re-locating a houshold together, joint bank accounts, and joint property ownership are just a few. You may find that when you and your college roomate moved out of the dorm into that new bachlor pad, and chipped in for the big screen TV it was more of a commitment than you thought.
No, this probably wouldn’t hold up in court, but it is amusing to think about…