Actually, there is no definitive time period to establish a common law marriage in those jurisdictions which recognize them, and it cannot happen by accident. There must be (1) An intent to be married; (2) the couple must hold each other out as husband and wife. This can be done by the wife taking the husband's last name, or referring to him as her husband. Finally (3) the marriage must have been consummated. If/when these requirements are met, then there is a common law marriage which is every bit a marriage as if it were celebrated in Westminster Abbey. By the same token, if one enters into a common law marriage in a state which recognizes the same, under the Full Faith and Credit Clause of the Constitution, the couple are legally married in all 50 states. It can only be dissolved by divorce, not be separation alone. Similarly, if one is a common law spouse, one is entitled to the same inheritance as if the marriage had been celebrated by an officiant. So, the issue here is not the length of time, but rather if the couple intended to be husband and wife, and if they lived in a state which recognizes common law marriages. If the answer to both questions is yes, then the surviving spouse is entitled to a spouses share by law. If the answer to either question is no, then the survivor can only survive if the decedent left a valid will with a specific bequest which will withstand challenge.