There's currently no schedule for Plasma 6. However, it has been decided, that Plasma 5.27 will be the last release in the Plasma 5-series. After that, work on the first version of Plasma 6 will be the focus of the development. Plasma 5.27 will first be released in early 2023 and will receive bugfixes, likely at least until Plasma 6.0 has been released.
A schedule for the first release of Plasma 6 will be added as soon as possible.
You can keep track of the progress of porting Frameworks and Plasma to QT6.