The short answer is about 45 minutes. The long answer is maybe 100 hours. Someone once asked Picasso how could he charge tens of thousands of dollars for a sketch that took him only 10 minutes to draw. His response was “That sketch took me a lifetime to draw.” I paraphrase.
I’m no Picasso. Plus any cache in the brain, save some technique and linguistic phrasing, stays in the brain. Every brand brief is a like snow flake. Each brand brief is built from scratch; leaning heavily on customer care-abouts and brand good-ats. All that information takes time to amass. I’ve taken months to write a brand brief. I’ve taken weeks.
In some cases multivariate statistical analyses were used. And slopes were plotted. Findings clustered. Interviews by the hundreds. Others have been developed on a shoe-string.
On most proposals I say it takes a month to write a brand strategy brief.
Now, to the next question: How long is a piece of string?