Holly-
Difficult to answer your queries as there is no 'typical'- I have worked out of London every week of my 2 year career with PA... but a girl who started at the same time as me has been on a long term london job. I have worked for 15 clients in 2 years, the other girl for 1.... Work life balance is what you make it- as all consultantcies, you work long and hard when the client has a problem, but when you're off the clock you can slope off a bit early and noone really minds.
My advice would be go with your gut feeling- do you like the people you meet at interview, for example? PA has a very distinct culture- quite a lot of eccentricity is tolerated, and I'd echo the point about the meritocracy (above) and people who 'fit in' love it and thrive here- people who don't hate everything about the firm.
Good luck- hope you decide to join us!