Support for Monterey will "officially" end when Sequoia is released. At this point no one knows the exact date.
That said, despite the end of "official" support, but based on history there are likely to be additional security updates for another 1.5-2 years.
For example,
Mojave - support ended Oct. 2019 but the final update was July 2021
Catalina - suport ended Nov. 2020 but the final update was July 2022
Big Sur - support ended Oct. 2021 but the final update was Sep. 2023
Note, however, that some app vendors will end support of their apps on Monterey and no longer issue updates as soon as Monterey official support ends.