Boxers or Creative Briefs

I think I need work on creative briefs. I can see the usefulness and necessity of a creative brief but I just don't think I'm so great at creating one just yet. A question that I feel like I should have asked during the lecture portion of the creative brief is " how long should it take to write a creative brief?" I realize that there are different types of creative briefs and different levels of details that can go into a creative brief but I didn't explore the idea of the time it should take to complete a brief. Clearly, one wouldn't want to spend the whole time frame for designing then entire project on just the brief and conversely one wouldn't want to under research and make the project a lot harder than it needs to be. I think with practice and will come the balance for becoming a creative brief master.