Development traffic on the mailing lists is diminishing lately, probably as vacation season is approaching. Therefore, there are no new updates regarding release schedules.
There is a discussion involving a backwards compatibility break in 5.5.0 relating to qDebug and non-latin1 character sets.
Bug report: https://bugreports.qt.io/browse/QTBUG-47316
This behavior will likely be reverted in 5.5.1.