If you are concerned about ACV going bad, ask yourself if tap water (that is treated with Chlorine to kill mold, bacteria, fungi, and all sorts of other germs goes bad if left out in the open too long.  Of course tap water reverts to it's natural nasty state if left open too long say in your chickens drinking fountain  BTW, Chlorine is a much more effective sterilizer than any vinegar known to man or beast but its health benefits are fleeting and ACV or any other flavor of vinegar's heath benefits will prove to be just as fleeting if not more so.  Go to your refrigerator and look at a jar of pickles and see it the label on that jar of pickles doesn't say "REFRIGERATE AFTER OPENING"  Now ask yourself why is that, if Vinegar is such a perfectly healthy preservative?   
There is even a good argument for adding Roundup to your chickens water because all vinegar is an effective herbicide the same as Roundup is an effective herbicide.