Three months between release of an API version and sunset is insane to me. Luckily we are an agile startup and can switch to a new API version fairly quickly (when there aren't breaking changes, which sometimes there are). I have no idea how a bigger company who has to go through layers of QA, release planning, and red tape could possibly keep up. By the time you develop and launch support for a new version of the API you would literally have to start all over and work on upgrading to the next version.
You can see the release and sunset schedule on this page of the docs: https://developers.google.com/google-ads/api/docs/sunset-dates
Here are a few of the release schedules:
Version: V7 Release Date: April 28, 2021 Sunset Date: September 8, 2021
Version: V8 Release Date: June 9, 2021 Sunset Date: November 3, 2021
Version: V9 Release Date: November 3, 2021 Sunset Date: February 9, 2022