I have background of 1-2 years javascript, 2-3 years C#, 4-5 years Unity.
How much time to take to learn C++, UE4, Blueprint each for make minecraft likely voxel, cubical world?
We are all learning 
This is a lifetime commitment; But that doesn’t mean you can’t finish any games while you learn.
I have similar experience with Unity and took me around 3~4 months to feel confident with UE4 and begin developing a game seriously.
‘Minecraft like’ game has little to do with engine or tools you use though; it’s more about your own design decisions.
Depends on how much time and passion you have 
It took me around 1,5 months to get used to the UE4 and Blueprints (but I had UDK experience).
Learning C++ will be easier for you as you already have programming experience
anywhere between 2-6 months,