Wait, RTM is "Release to Manufacturer" as in the final release build of Windows 8. If you have RTM installed and an installed RTM key, there's no expiration that I'm aware of. Sure, the Developer Preview, Consumer Preview, and Release Preview builds have an expiration date, but the entire point of RTM is that the code is final and the licensing is for the final release.
I've been running RTM with a MAK (mass activation key) through my company's Microsoft partnership since the release date (August 15), and it IS the final build and same version people will be buying in stores now.
If you are already running RTM, there should be absolutely no need to reinstall or do anything to have a working system forever provided you have a valid license key for Windows 8 RTM Final.
*EDIT*
I just confirmed the downloadable version of Windows 8 from the MSDN subscirber downloads section is dated 8/15, so there is absolutely NO difference between the retail code and RTM code. I see absolutely no reason that a retail key would not work on an RTM copy or vice versa provided the Windows version is the same (IE enterprise vs professional vs home or whatever flavors they have now).