For the record, I have yet to have any flavor go bad - even after 6 years (and counting). Also to note is that I mostly get 4oz bottles and then transfer those into amber glass bottles with polycone caps.
So in my experience, flavors just don’t go bad. The most that could end up happening is potency loss, but I haven’t noticed such happening… Likely due to do proper glass-bottle storage.
Whatever you do, NEVER put flavors into those glass bottles with the rubber droppers tops! The material is too porous, not to mention some flavors will eat through it and get contaminated. ONLY use glass bottles with POLYCONE caps for long-term storage. In order of what color glass bottles are best, in descending order, it would go: black, amber, cobalt-blue, clear. I’ve seen green too, but I’m not sure where that lies on that scale - probably near cobalt-blue, I’m guessing. Of course, I keep all my flavors in the dark anyways - but not refrigerated, just apartment temperature.