With KDE Plasma 5.6 released, the developers are now making plans for Plasma 5.7. According to https://community.kde.org/Schedules/Plasma_5, the major release will become available starting with July 5, 2016, three maintenance releases for KDE Plasma 5.6 being scheduled until then.
The repository freeze will be reached on the June 2, the final freeze on June 16, when the beta version will get released as well.
After that, KDE Plasma 5.7.1 will be released on July 12, KDE Plasma 5.7.2 on July 19, KDE Plasma 5.7.3, on August 3, KDE Plasma 5.7.4 on August 23and KDE Plasma 5.7.5 on September 27.