Common law marriage is not recognized in most states (USA) today. So no matter how many years a couple lives together, they don't have to worry about common law marriage.
States that do recongnize common law marriage: Alabama, Colorado, District of Columbia, Georgia (if Made prior to 1997), Idaho (if created before 1996), Kansas, Montana, New Hampshire (for inheritance purposes only), Ohio (if created prior to 10/1991), Oklahoma, Pennsylvania (if created before 9/2003), Rhode Island, South Carolina, Texas, and Utah.
The following are FALSE statements about common law marriage:
Cohabitation alone does not constitute a common law marriage
The property bought by a common law spouse will be split half and half in the event of separation
Fact: Rights to protecting a family residence and dividing family assets are only granted to legally married couples
If a couple has a child together, they must adopt him/her
Should a common law spouse die or become disabled, all assets automatically go to the surviving spouse
There is no such thing as common law divorce
Fact: if you are married by common law and then decide to end the relationship, you will still require a legal divorce.