A couple of consultancies have mentioned that their typical working pattern is three nights away, fourth day on client site, fifth day at home or office. How realistic is this in practice? How many weeks will turn out to be exceptions (i.e. away from home before 6am Monday or after 9pm Thursday)?

As everything else in consulting, IT DEPENDS. I've been based in London for the past 3 years, never spent a night in a hotel. A few of my colleagues have been commuting to Switzerland every week for the past 10 months, getting there Sunday night only to return on Friday night. I guess most people are somewhere between. If you're seriously considering a career in consulting, you need to stay flexible, and the easiest way to achieve that is by not making up any silly rules (like 3-4-5). It all depends, on the client, on your boss, on the project, on your company policies, on the type of work you do, everything. It all depends.

Agree totally - Nothing is constant in a Consultant's life. In my 5 years with a big Consulting firm , I have had all kind of situations. I am based in London.I have done Monday early morning -Friday late evening in Manchester for a year, I have done London for almost 1.5 years at a stretch, I have done Mon-Thurs in Newcastle and Friday from home for around 6 months, 1 year in Reading(travel time approx 4 hrs every day) and so on. The rule is that there is no rule.