Hurry up and use it all. Then go buy some more if you liked the results. Our hobby needs to show retailers that we are customers and consumers, even though we may not be buying ammo.
For inventory purposes, I generally scratch the month/year of opening on metal cans and put a big X on the container to show that it is in active use as of that date. Keeps me from opening a fresh can before I use up the oldest batch. Same thing for the plastic jugs, only with a Sharpie. I have some 8 oz. and 16 oz. powder containers from the 70s that have been fun and interesting. Not fun enough, though, cause I buy a big (4 lb, 5 lb, 8 lb) supply of REALLY good powders while they are still available. See 1st paragraph. I think that the way powder is sold (tight, airproof containers) and the way we should keep it (at room temperature, avoiding humidity) they may be safely used for decades. Sniff test should be done each time you open it. Vinegary? Not good.