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As long as they're kept airtight in dry, cool place....I imagine for as long as they stay moist. They're not really going to go "bad" with the amount of sugar that's often in jellies.
My fruit jellies last about a month, no problem. I can't say how long they will last otherwise because they are always eaten by then. You need to be sure your storage is absolutely airtight though, because all that sugar will attract moisture. If you keep an eye on them, you can tell when they are getting a bit damp. At that point, just remove them and toss them in a bit more sugar and let them dry out a bit before repacking them in the container. I also prefer using regular granulated sugar instead of sanding sugar, but that can depend on the size of the granules you prefer. I notice the term sanding sugar is used to describe everything from a really fine grain to a slightly larger one. Larger grains give you more sparkle, and a bit more crunch. I also recommend you store the jellies in layers separated with waxed paper. It makes removing them a lot easier, should the need arise. If you don't separate them that way, and they do get damp, the jellies can all stick together in a big mass that is difficult to impossible to separate. As long as you keep them fairly dry and coated, I would think they would last quite some time. Bacteria has a hard time living in high sugar environments, which is why it works so well as a preservative for things like candied peels and fruits.