Probably not too long. I wrote (a crappy version of) Breakout in my second semester of high-school programming, and that was using Pascal plus some homebrewed x86 assembler for the graphics (both of which were a nightmare that I wouldn't recommend to anyone), so simple games clearly don't require any deep knowledge of the discipline; if you've got a computer-engineering background already and you're working with a modern graphics library, I'd call it a couple weeks of casual study. Less if you're using a game-specific framework, but those skills don't usually transfer well to other things.