One resource would be the Product Life Cycle page, showing the length of support (General Availability, Extended Support, Mature Support, and Retired) for each version of ArcGIS for Desktop. This is what's currently listed for ArcGIS for Desktop 10.8:
- General Availability: Feb 2020 - Feb 2022
- Extended Support: Mar 2022 - Feb 2024
- Mature Support: Mar 2024 - Feb 2026
- Retired: March 01, 2026
