Closely related to, except that's about when the branches start LTS, this is about when they are designated as being at end of life (EOLed).
For reference 5.x EOL was on the day 7.x was released.
Drupal 6 EOL is three months after 8.0.0 release.
The Drupal 8 LTS (Long Term Support) will be released sometime between the opening and release of 9.0.0
Things to consider:
1. How many major core branches are supported at once? The current LTS policy says 2 with a 3 month crossover, is that a hard maximum?
2. If we end up with two LTS releases of 8.x for any reason, would that affect the number of major branches we'd want to support.
3. Do we expect people to migrate from 7.x to 8.x when the migration is available, or wait for 9.x? When will the 7-8 migration be available?
4. Do we expect people to migration from 8.x to 9.x (especially if that's a smaller jump) or wait for 10.x?
5. If 9.x ships with a fully support migration path from 7.x, how much time is reasonable to allow people to jump if we support that?
6. Do we want to support going from 7.x LTS to 9.x LTS, or just to 8.x LTS and 9.x minor releases - the answer to #1 affects the feasibility of this
7. What's going to happen with D6 extended support? How quickly with 6.x usage drop off on http://drupal.org/project/usage/drupal? Will we finally see the remaining hard-core 5.x sites disappear?
8. Given we've shifted to a timed release cycle for new releases, would we consider moving to timed EOL, for example LTS + 2 years? Instead of pinning it to major version + 2 release dates which is more variable.
(add more if you have them)
A few examples of what we could do:
Drupal 10.0.0 + 3 months (same as 6.x EOL timing)
Drupal 10.0.0 release day (same as 5.x EOL timing)
Drupal 10.X.0 LTS release day (allows for LTS to LTS)
Drupal 9.X.0 LTS + 1 year (assumes we start only supporting version N to version N + 1 again and make the upgrade path smoother.
Drupal 8.X.0 LTS + 2 years (allows us to announce the actual date much earlier than any of the other options, because it's only based on the 8/9 release cycle).
Add more options here.