For how long is the RMC ID kept?
There is no time limit for the RMC ID. If a new version of the RichMedia Engine is installed “over” a former RMC, the RMC ID will be kept. On the other hand, if the current RMC is uninstalled before the installation of a new RMC, then the RMC ID will be different (a new one will be generated server-side; see previous answer).