It is hard to say, It depends on a lot of variables – Experience, level of detail required in the character, will there be hair, fur etc, detail of texturing, rig requirements, are you building the rig from scratch or are you using an addon like rigify, is a facial rig required, how much free time you have a day to spend on it… the list goes on…
For example, it could take an experienced artist a matter of hours to create the simplest of characters, whilst an inexperienced beginner may take weeks or months. A lot of time for beginners will be taken up by learning how to do it, rather than the actual execution.
I am myself , in the process of creating two characters for personal projects, one for a game, and one for animation, and both are in their third iteration because I have learnt how to do them better each time and have started over twice now, and this process has taken me most of this year, but a huge amount of that has been learning.
I would recommend you check out some of the CG cookie character courses , specifically one that suits your needs for your project and work alongside it. Usually you can make your own design so If you know what you want to make you can work through a course and make your own character.
Im not sure of your skill level, but this course came out recently and covers the entire workflow of modelling a character for use in Unity so maybe you could work through it to create your game character? And I believe that a texturing course and others will follow. https://cgcookie.com/course/modeling-a-robot-game-character-with-blender