Casey has stated on stream that the win32 platform layer can be used anytime (though contact Casey for approval first); however, the game code is licensed for 2 years after the release of handmade hero.
The license is on his front page:
How will the source code be licensed?
Two years after the final version of the game is released, 100% of the source code will be released into the public domain. Prior to that, all rights are reserved, so please contact me for permission before you use it in your own projects. This is a precaution to prevent people from directly cloning the game until after it has been out for a reasonable time, since this project is an expensive endeavor and I hope to offset some of the cost through sales of the game proper.