Over the years I have done a lot of research on this subject. I think this article from http://shelflifeadvice.com/content/how-long-will-they-really-last-part-I gives a good summary about the expected shelf life of spices and herbs.
What about dried herbs and spices? According to the experts at the Spice House in Evanston, Illinois, the shelf life of these dried products varies quite a bit depending upon the type of spice and the form it’s in. For example, peppercorns may last 4 years, but poppy seeds will barely last a year. As you might expect, whole spices will hold up longer than ground spices. As dried spices and herbs age, they oxidize and lose flavor. However, that doesn’t mean they’re dangerous or spoiled; it just means that you need to use a greater quantity to get the taste benefit you got from a smaller amount of fresher product.
I think a good rule of thumb in checking for freshness is if the smell is there then the flavor will be there.
To prolong the shelf life of your spices store them out of sunlight and away from heat.
Transfer your spices to glass containers. They will help preserve the freshness of you spices longer than plastic containers.