Most spice reference books, including spice bible and complete book of spices say a year. But, this is said because it used to be that most herbs and spices are harvested once a year. This isn't always true with modern shipping methods. A year is the general rule of thumb though.
Just remembered. While in culinary school during a presentation by a McCormick rep he made the most profound statement. He said that once peppercorns (black, white, red or long) are ground it takes just 90 minutes before the essential oils start to breakdown.