Maintenance and release schedule
You can find the detailed schedule for major and maintenance releases at: detailed schedule.
Major releases
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.
Major releases are planned to be actively maintained for 12 months after their initial release. For long term support options check out the Nextcloud Subscription offered by Nextcloud GmbH.
Maintenance releases
Maintenance releases are scheduled in a 4 week cycle with one week before the release date having the freeze and RC 1.
Critical changes
You can find important documentation for app developers here: https://docs.nextcloud.com/server/25/developer_manual/app_publishing_maintenance/app_upgrade_guide/index.html Each document lists a link to the breaking changes of the corresponding release.