All of the epoxies we use at work have a DOM, date of manufacture and a DOE, date of expiration. They are 6mo and 1yr respectively, it rarely expires because of the sheer volume we use daily. Heck, even our tapes, both release(teflon) and masking, we use have expiration dates. It's all controlled and heavily monitored and a huge pain in the keester.
I buy the smallest kit I can. I am lucky though, I live and work in an area with two major aviation manufacturing companies, a broader area with a half dozen more major aviation manufacturers and dozens of mom and pop certified repair facilities. There are several local companies that supply adhesive to these aviation companies and to automotive and marine repair shops , a couple of these carry both the Devcon and MarineTex. I can walk in a buy a kit that is fresh and the stock has been rotated. I have purchased several kits of Devcon that were expired or would not properly set up. I did learn a trick to get the heavier base materials to become more pliable. I use a homemade T-pin with a 6" shank. Put it in a cordless drill and begin remixing the base epoxy, you will be surprised that it becomes more pliable, but one thing I have noticed is with that process, the pot time, the time you have from the minute you mix up the kit is relatively shorter and the product begins the exothermic process faster. If you really want to speed it up, add in the catalyst and use the T-pin to fully mix the adhesive, you best be ready to apply and get everything together, because you will not have much time to dilly dally. I don't know why when thoroughly mixed like this the process begins so quickly. We generally use the tongue depressor method and the pot time is usually about 30 minutes, some 60 minutes, you can extend it slightly with a good stir or two during the application.