I had posted this thread a few days ago but it must have got deleted, so I'm re-posting.
I work for a strategy consultancy in Europe. We generally work 12-14 hours a day in the office (no home-working, except the odd w/e.
I personally consider 8-12 hours to be optimal due to following factors:
1) People who are fresh and rested perform much better and can be really creative. Medical studies show that you achieve peak performance, four hours after you wake up. They even suggest that you nap for a couple of hours to hit that peak twice a day.
2) Exhaustion increases churn which increases recruitment and training costs and reduces efficiency. Some turnover is unavoidable as people seek better opportunities but do we need to exacerbate it?
3) Exhaustion impacts people's health, making consulting increasingly unatractive as a career, as people become more sensitive to work-life quality issues.
4) Lack of free time impacts people psychologically as they miss the normal pressure-release valves, turning them to potentially unhealthy behaviour, e.g. excessive drinking, extramarital-affairs, or generally just turns them into a**es. We see plenty of those on this board :)
5) In most cases working 12-14 hours is unnecessary. It's the result of bad project planning, a mis-placed focus on quantity over quality (Is it really sensible to produce a 400 slide set to tell a 40 slide story? Couldn't 100 slides suffice?), or a case of macho attitudes.
I should say that all my arguments are based on the assumption that a 'use 'em up, spite 'em out' business model is unworthy in all ways - bad for people and bad for business in the long term.
I'd welcome any thoughts from all who have similar experiences.