Most people have a romanticized view of management consulting work. Usually wrong. I wanted to talk about my typically day/week as an associate (MBA level) consultant, and particularly how that changed as I moved up the ranks to the partnership. It will help you understand why mental math is so useful, as well as the extreme stresses of the job.
Keep in mind that I was a very, very driven partner. It was very normal for me to work most of Saturday and Sunday and I typically traveled on a Saturday night so I could arrive in a new city the day before the week began. This allowed me to adjust and prepare adequately for the week ahead. Many partners could, and do travel less. I still maintain an aggressive work and travel schedule and have never formed a family.
It all depends on priorities and client demands.