Technically the liquid should not go "bad" as it is unlikely to harbor pathogens. PG is considered a preservative because it inhibits bacteria growth. However, over time the liquid will begin to break down. It will loose its effectiveness, may begin to harden as water evaporates and may oxidize. The flavor will also change as the ingredients degrade. The manufacturer of Super Cigarette claims a shelf life of 24 months if kept in the refrigerator.
However, refrigeration is not needed. Just a cool, dark place. And do not open packages needlessly.
24 months from what date is a really good question. But if a package remains unopened and is properly stored it could, theoretically, last indefinitely. I suggest simply marking the package upon purchase and using it before the recommended time is up.
