The best thing to do is follow https://azure.github.io/AppService/ in addition to https://github.com/Azure/app-service-announcements @Anonymous with regards to when a supported runtime has been released or scheduled to be released.
To answer your question with regards to how periodically, there isn't a specific every x months or every Xth quarter. Even as runtimes are released by providers (NodeJS, PHP, etc.) doesn't necessarily mean those runtimes will be immediately released on the App Services platform.