How Long Should Support Be on a Supported Device?

I saw an article recently in the register about small rooters that have got a critical vulnerability, but they're not going to get patched. So some people may have bought them three years ago, and now they have to just scrap them. That doesn't seem right to me. What is the amount of time that should be acceptable for the vendor to drop support for device? Should every device you buy have a best before date on the box so when by it, you know that after that date, i shouldn't expect support any more? I don't know that there's an easy answer to that. But i definitely do like the idea of saying, definitely supported until whatever date like on the