Maintenance and release schedule
You can find the detailed schedule for major and maintenance releases at: detailed schedule.
Major releases are typically scheduled once every 4 months with the first 10 weeks being the development phase followed by freeze phase with four beta release, two RCs and one final each one with an interval of 1 week. Specific dates for each release can be found on detailed schedule.
Maintenance releases are scheduled in a 4 week cycle with one week before the release date having the freeze and RC 1.
You can find important documentation for app developers here: https://docs.nextcloud.com/server/latest/developer_manual/app_publishing_maintenance/app_upgrade_guide/index.html Each document lists a link to the breaking changes of the corresponding release.