The TYPO3 Core Team releases LTS-versions (long-term support) every 1.5 years. These versions are actively maintained and supported for 18 months and receive security fixes and critical updates for an additional 18 months. This results in a total of 3 years of free support by the TYPO3 community.
As LTS-versions are released every 1.5 years but are supported for 3 years, there are always two stable TYPO3 LTS-versions available that are actively maintained and kept up-to-date and secure without breaking changes.
A release series consists of a number of sprint releases, starting with “0”. TYPO3 version 9.0 and version 10.0 are typical examples. New sprint releases are published every 8 to 12 weeks until the LTS-version is released, which marks the end of a release series. Sprint releases are bleeding edge but at the same time well tested and stable enough to be considered for new projects. An upgrade between two sprint releases (e.g. version 10.1 to version 10.2) is considered trivial and should not take very long.
TYPO3 v9 LTS (also known as version 9.5.x) was released in October 2018. The next LTS version will be TYPO3 v10 LTS which is scheduled to be released in April 2020.