@jdsilva is correct that there is no set schedule. While that can be frustrating, it's also an advantage in some ways. But there is a definitive process through which firmware must evolve, and depending on customer adoption rates and the level of field exposure of new firmware, the timeline of that process is going to vary. It will vary from product to product, as well as very a bit from release to release in the same product line.
There's a recently updated firmware best practices guide on the docs site, and has the latest updates on GA, SRC, Alpha/Beta releases, etc. Check this out if you haven't come across it yet, it's a good read and was refreshed late last year. https://documentation.meraki.com/Architectures_and_Best_Practices/Cisco_Meraki_Best_Practice_Design/...