Unity In A Nutshell
How come I've Never heard about it until now?
How Long does it take to make a game?
To make a game is like asking how long a piece of string is. Its really impossible to look at a game and estimate on the spot how long it will take to make. All that is evident is that no matter how simple the project might seem on paper it will take much longer then you except even if you have the necessary experience and skills.