>My understanding: Living together like a married couple long enough conveys actual marriage in some places in the US, various rights some other places in the US

Intent and agreement is a big one, not length of cohabitation. The two both must consent and intend on being married and basically tell everyone they are married. Just cohabitation for a long time isn't enough. Nobody can suddenly "find" themselves in a common law marriage they didn't intend or want to enter into, they both must intentionally agree to enter into one.

http://en.wikipedia.org/wiki/Common-law_marriage_in_the_Unit...