What is this 10-year rule?

A very simple answer to this question is that if you have been married for over 10 years, the court may retain jurisdiction over the issue of spousal support, and you may be entitled to support of indefinite duration.  Derivative social security rights also arise from a 10-year marriage.

However, many many many factors affect the outcome, so I would make sure you ask your lawyer before assuming you are paying or receiving lifetime alimony.  This article may also help: 10 Year Rule Much Ado About Nothing.