Dried spices can be stored for a highly variable amount of time - garlic powder will probably hold its flavor for a long time, but coriander loses flavor fairly quickly.
Most chefs say you should only keep spices for six months, then they need to be discarded. Most dried spices usually will lose some flavor after that time.
The answer partly depends on how fresh they are when you bought them and the exact kind of spice. The cook book I have recommends that they should not be over six months old for the best flavour.
Spices don't go bad, unless they have natural oils that turn rancid. These usually have expiration dates. Of those spices that don't have oils and don't spoil, they do become less strong over time.
