I think it depends on a variety of things. Mostly the current market affecting the amount of work to do and the department you are working in. I do think you can expect to work a lot more in a magic circle firm than any others, particularly when the country is in an economic 'boom'. However, like others have said, a lot of people will leave after 3-5years pqe. I read an article that a lot go into top regional practices, i.e. Eversheds, Addleshaw Goddards etc. because the work/life balance is better and you get better value for your money when buying a house.
If a firm is paying you £40,000 straight out of uni, I don't think you can sit back and relax too much on the job.
...and how to stay positive