55-60 hours would be fairly normal for a large firm partner, although Mike is spot on when he says that it's mainly meetings, travel, reviewing reports, reading, etc. The average partner would arrive at say 7:45 and leave at say 6:45, so that's 55 hours pw, and then add on the occasional piece of work at night or on the weekend. You'd also have the occasional conference call with colleagues in Europe or the US, which would either be around 11pm or 6am, but this would normally be done from home.
Partners in certain parts of the business would be more involved in the detail, for example those who work on transactions or corporate recovery work would frequently work past midnight and occasionally through the night. (It's not fun, especially when you're no longer 22 years old, but if it has to be done then you just do it)
Thanks for the reply. A friend of mine is a grad at PWC, doing the long hours and aiming high, so I was just curious about this.