Under commonwealth law how long do you have to be with someone before you have to pay alimony if you separate?

You could live with someone in Kentucky for 60 years and not have to pay alimony after separation, if you never married.  Like most states today, Kentucky does not recognize common law marriage.  However, if you became common-law-married while living in another state, under its laws, you would be considered married in Kentucky.

The length of a marriage is one of the factors involved in deciding whether alimony gets awarded, in a divorce;  permanent alimony is rare for marriages of less than 10 years, although shorter-term awards, often called rehabilitative alimony, are sometimes given to help one spouse get job skills and back into the workforce.