How long were you learning c++? And where did you learn it from?
3 - 4 months, Internet.
You never stop learning C++. But I started a year a go.
It’s like learning english… or learning french, spanish, or japanese 
You have to study and practice forever :v
For example, the main reason I read and write on many dev forums is because I have to constantly practice english. Still not really fluent.
Then when did you know you were ready?
I have learned everything I know through the internet, on places like these forums.
I mainly work on my own projects, so I don’t need to be “ready” in any sense, I just learn what I need to know, as I go.
[USER=“328040”]Dwarf Killer[/USER] there are lots of different things to worry about… if you are expert in C++ and knows nothing about game industry: meshes, materials, textures, effects or even math, you can’t do anything.
While producing a game in UE will have some benefit if you are doing things in C++, you need to learn a lot of things prior C++ to be enabled to produce a game. I tell this for my own experience because I had 25+ years as C++ developer and when I jumped into the game industry knowing nothing about it, I realized that a huge learning curve would exist for anyone, not just me, with no prior experience making games, modeling, animation, FX, etc.
Unless you know already a lot of stuff in any game industry area and are just diving into Unreal, I would learn first how to use everything inside it: materials, effects, blueprints, animation blueprints, AI logic, and only after that I would say you are ready to learn C++ oriented to producing games into UE4.