Heat, light, and oxygen are the big enemies of all spices, but of ground spices in particular.
One year is the generally accepted lengh of time for ground spices to remain viable. How you store them affects that, of course. And, obviously, "in general" means a particular jar can last a longer or shorter time.
The question arise, too, "how long from when?" That jar of ground cumin you bought "fresh" may have been sitting around in a warehouse for six months before even getting to the supermarket shelf.
Note that all of this refers specifically to ground or otherwise processed spices. Whole spices, in general, can last many years. That's what made the spice trade possible (often enough spice caravans lasted two-three years or more).
For an effectiveness viewpoint, you should be buying whole spices anyway, and grinding them yourself. That's how you assure maximum potency as well as shelf life.
They have taken the oath of the brother in blood, in leavened bread and salt. Rudyard Kipling
