Artful Dodger:

>  When is the cutoff?  You date a girl named Jane for a week and then move in with her.  You live together for a month ... etc.  

Depending on where you live, the law defines common law marriages, domestic relationships, etc.  In many places the law is clear.  For example, in Canada the definition is clear:

a) the couple have been living in a conjugal relationship for at least 12 continuous months;

b) the couple are parents of a child by birth or adoption; or

c) one of the couple has custody and control of the child (or had custody and control immediately before the child turned 19 years of age) and the child is wholly dependent on that person for support.

Some provinces also include a further definition such as both partners owning together property or financial assets.

In the United States 11 states and the District of Columbia recognize common law marriages.  26 states recognized common law marriages in the past but no longer recognize them.  13 states never recognized common law marriages.  The definition varies from state to state, and often under different conditions for different states.

So it is more than a matter of just "shacking up together".  Legally speaking a couple can find themselves receiving all benefits of married couples in one state while in other states they would not.  Earlier I said that in some cases common law couple can find themselves discriminated by the law.  By this I mean that not all couples are treated equally.