The short answer is a lot.
During my time at McKinsey, I worked an average of 75 hours a week when on a project and my experience was pretty typical. That number does not include travel, which can easily occupy another 10 hours a week.
Until you've lived this life, 75 hours a week may not sound like a lot but for me, it was. It's 0800 to 2200 Monday to Friday and then 5 hours on Sunday evening (this was my working pattern). It is very difficult to have a life outside of work when you're doing this and this was a big reason why I eventually left consulting, despite the fact that I was being paid very well.
Of course, there are peaks and troughs. The most relaxed project I worked on had me working about 60 hours a week. In addition, as a consultant you'll occasionally find yourself 'on the beach' (consulting lingo for being between projects). This means that unless you get roped in to helping with a proposal, you don't have to go to the office at all. It's great but rare.
The worst project I worked on involved working 100 hours a week (0800 to 0000 Monday to Friday and 10 hours each day Saturday and Sunday). This was not unusual- more than one of my friends did a project like this for several months and can happen to you irrespective of what role you're playing on the team.
Overall, being a consultant is hard work and if you're applying you should be prepared to work very hard for the next few years. It's tough but worth it.
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I'm a ex-McKinsey London EM who recently left the Firm