From my experience, you'll realize that you'll need to use both. The ratio would depend on your resources, ROM/RAM, hardware peripherals and TIME! There are some stuff that are very tedious to do in assembly and there are stuff where it is inefficient in C. I guess these are the things you just learn from experience. But man, you have a month to do this job and you don't even know what the task is? Like somebody mentioned, hopefully it is not a big project.

-John G-