Qt 5.5 released July 1, 2015 was declared by Qt Company as semi-annual release. Next release Qt 5.6 according to currently available release plan should be available in early December 2015. But all previous releases usually were delayed for several months, we will see how this plan will come true.
According to this Qt 5.6 will be long-term support release and will still support C++ 98 features. As for Qt 5.7 it will be something completely new, moving to new compilers (MS C++ 2012 and GCC 4.7 will be required at least for compilation) and certain C++11 features will be also required.