I go with 2 years AT ROOM TEMPERATURE - 3 if vacuum sealed and in the dark and, 5 if frozen at -10 (US) or lower. No science to back it up, just my experience with various foods, flavors and chemicals I keep for other uses.
Now flavor is another matter, steeping is good for some, and some degrade quickly, that's just a matter of knowing how your flavors act and, trial and error to preserve them as long as you can. Like my peanut butter - has to stay in the fridge except the bit in the tank, it fades in 72 hours at room temp.