I think there isn't an official one, but since 2016 there's been a major version released approximately every two years, and a minor version every couple of months.
While a set-in-stone release schedule doesn't exist, there is _a lot_ of development activity (emacs-devel is a very busy mailing list). 99.9% of the time it's safe to use the bleeding edge head of the repository if you want to use the latest stuff.
except in the few months following a major release (at least for me)