Shelf life varies from one herb to another. A powdered herb will lose its properties much faster than a whole or cut herb. Those with volatile oils like peppermint will lose their freshness much sooner than those without the oils. Roots have the longest shelf life as they are much denser than flowers or leaves.
So it's rather difficult to give a quick and simple answer. Most herb books have something to say on the subject but they are not very specific. Just as we did, you will gain insight in this area as you get more familiar with each herb.
In general, store herbs in air-tight containers, keep them as cool as possible, and don't expose them to light unless in use.