My answer is that there is no "industry standard" that I am aware of to which someone could go for any guidelines about shelf life or expiration dates of brazing pastes. Each manufacturer uses different criteria for setting their own shelf life or expiration dates for their brazing filler-metal (BFM) pastes. A number of manufacturers have stopped showing an expiration date on their paste containers altogether but instead show a "date of manufacture." It used to be common to find an expected shelf life printed on the containers, but because of rejections of good BFM paste merely because of a printed date on the container, some manufacturers have stopped doing that.
Here are two things that can easily be done to determine the usefulness of a "questionable" brazing paste when that paste is at, or has gone beyond, its published shelf life.
1. Call the manufacturer and get their specific thoughts about expiration dates of their paste. Ask them what they suggest regarding brazing paste that exceeds those dates. If you can, get a note from the manufacturer showing the maximum shelf life they would allow/guarantee for their pastes.
2. Test the "creaminess" of any BFM pastes you have that appear to be getting near their expiration dates. I used to do this by extruding a tiny amount of paste out of the end of the tube. As long as the paste was still extrudable and creamy, I would use it in our brazing shops. Obviously, for special aero-projects requiring use of only materials that were still within the printed shelf-life guidelines on the containers, I could not do that. No "expired" material should ever be used on such projects no matter how good the paste might still be. With today's traceability needs, any use of BFM paste that had officially "expired" or gone beyond its printed shelf life could cause the brazing shop to put themselves in a position where end users would have an opportunity to point their blame finger at that brazing shop should something go wrong with the brazement in service, even if the BFM paste had nothing at all wrong with it!
Having said that, I have had no problem using BFM paste in a variety of noncritical commercial brazements as long as the BFM paste is still creamy and extrudable. Please understand that the filler metal in the brazing paste does not suddenly decide to go "bad" just because it has reached its expiration date. In fact, even if it is older than the printed shelf life on the container, it can still be finished. It's not like food, where expiration dates should be carefully followed for health reasons. Brazing paste expiration dates merely represent the limit of a manufacturer's guarantee to replace the materials if it should separate out from its binder system and settle in the bottom of the container, thus becoming more difficult to work with.
Here's another important point about BFM paste, whether it's somewhat new or is past its so-called expiration date. Should the paste seem to be drying out, the plastic cartridge (or pail) that is holding the paste does "breathe" to some extent. This allows some minor amount of air exchange (through the walls, around the end piston, etc.), which can cause paste to dry out. The BFM paste can usually be reconstituted to a creamier state by mixing in some extra "binder," which you can buy as a separate item from several different BFM suppliers. (It is NOT recommended to merely add water.)