While child support orders may be modified every 12 months, Property Settlements in a divorce are usually permanent unless the party wishing to modify the Property Settlement can prove fraud, duress, or mistake happened at the time the Property Settlement was originally created.
However, on September 9th, the Indiana Supreme Court held that if there is language like, "unless otherwise agreed by the parties," or "unless otherwise ordered by the court," in the Property Settlement, it CAN, in fact, later be modified if there has been a substantial and continuing change in circumstances for one of the parties. (Pohl v. Pohl)
In short, when getting divorced, pay close attention to where you are permitting the magic words "unless otherwise agreed" and "unless otherwise ordered" to be used in your Property Settlement if you want to ensure true finality.