Our first 4 week project at school, I used about about 5-6 weeks though, extra time both before and after the deadline. You can just keep adding so many details
When I was brain storming, I didn't know exactly what I wanted to do. I wanted to create something that required more texturing than modeling, since I was pretty inexperienced with that, so I had to incorporate that somehow. I found a picture done by Escher called "Relativity stairs" and since I was also playing Portal at the time I thought the project would be perfect to test my skills on.
Everything you see in the picture was made by me, I took loads of reference pictures from within the game, but ultimately I recreated everything myself to get the style that you find within the game.
This is the result, picture took approx 9 hours to render.
Note: Added a picture that shows some of the different stages how I created this picture [link]
Edit: I made a very large version of the image available for download (22mb). So people will be able to get their own print made. Just don't begin selling prints without my permission, or Valves for that matter.
Edit: Wow, daily deviation. Thanks a lot
And thanks for all the kind responses, always nice to see something you put this much time into get appreciated.