I doubt there's a good answer to "how long will they last?". The problem is that flavorings consist of such a variety of substances that it's difficult to predict how they might react. I'm thinking keep 'em cool (but not cold) in a dark place, and use wine preservation gas mixture to displace air (oxygen) whenever you re-seal the bottle.How long do you think flavorings will last, and what do you suppose the proper storage would be.