Of course the best thing you can do is check expiration dates on packages, boxes and vials. If you buy bulk loose powders (like tartaric acid in baggies from a larger sack) from your home winemaking or homebrewing supply store, be sure to ask the store owners when they opened the larger batches from which they re-packaged your smaller doses.
As a general guideline, things that have microbes as ingredients (yeast, malolactic freeze dried culture) pretty much need to be used within two to three weeks of opening. Powders that have lots of “delicate” ingredients like dry packaged complex yeast nutrients are really only good for about six months after opening. I find that inert dry acids like tartaric, malic and citric are pretty bombproof. I’ve had opened bags for over a year that have still been good. What seem to me to be the most unstable, apart from microbes, are liquid suspensions like liquid tannins, oak products, or fining agents. Usually these need to be used within a few weeks of receiving them, and certainly within a week or two of opening them. When in doubt, check a supplier’s website and don’t hesitate to contact the manufacturer.
As far as fermentation locks go, I tend to change the SO2 solution in my airlocks once every week or two. The sulfur dioxide will degrade over time and it’s nice to be sure that it is fresh and doing a good job of keeping down the microbes around the opening to your containers.
