As I am asking this question, I am a beginner. I want to build a 3d game that involves building your own vehicle with guns and fight. (Not a clone of robocraft it will be different).
I searched lots of tutorial videos but I don’t know where to start.
I want to use c#.
Where should I start 2D or 3D?
How much time should I put on a week to get a good result? (Average)
Start from the beginning here: burgzergarcade.com
Then move on to something like the RPG tutorial for deeper scripting knowledge. Then start looking around for more advanced tutorials 
Spend as much time as you can on learning. Combine with a treadmill if it would otherwise take away exercise time 
Unity has a learning section with video tutorials. I’d recommend going over them. Even if they don’t appear to fit your game concept they will still teach you how to work with the editor and basic scripting.
Everyone’s different, man. You may be able to absorb a ton of knowledge by just watching video after video of tutorials, or you may need to dig in to actually understand.
There’s no magical link anyone can provide you that is guaranteed to teach you everything you want to know. Just like anything, you just have to start doing it. Start anywhere. Do you want to learn 2D or 3D? The workflows are different in a lot of ways, but the core concepts are similar, too. So learn whichever one appeals to you.
Go through the official tutorials. Go through the unofficial ones. Heck, if you go another level up on the forums, there’s a whole board called “Learning.” Go there. In particular, check out the List of Tutorials thread. Google for specific things you want to learn how to do. If you discover something in the process that you don’t know or understand, Google that.
There’s no substitute for experience. Get in and start messing with stuff. There’s no wrong way to learn. Except by copying and pasting. Don’t do that.
thanks to all of you