Have you noticed that some solid fuel tablets have more smell than others?
Many such solid chemicals will sublimate or vaporize into the air, either by a little bit, or by a lot. The more smell you perceive, the faster it is sublimating. When you put the tablet into a plastic bag, you are limiting that sublimation into the tiny space of the bag. So, if you start with a fuel tablet that has little smell, and then if you keep it fairly well bagged, it should "keep" for a long time.
Some plastic bag material has large pores, so the smell can get out over time (so your fuel tablet will go stale faster). Good plastic bags have tiny pores and are called gas-tight. I have some.