As long as possible, as long as someone is using it, as long as someone can keep maintaining it.
If the main developer team can no longer maintain it then open-source it, put it in the public domain and set it free. Ditto for firmware and hardware documentation.
Companies oughta be forced to release all information they have on hardware they no longer maintain and disable any vendor-lock crap once warranty ends.
Yes hardware gets old and in the computer realm it usually means it’s rendered obsolete, but that doesn’t mean it doesn’t have its uses.