It all depends on the firm and the contract you are on. I worked for a company that was quite fair, tried to shift jobs to the following day where possible. I actually earned very good money while doing it, especially in the summer, money for nothing.
I've heard of people getting the pee taken out of them though, basic using it as additional hours to book in servicing etc!
I think you're legally entitled to not have to work a certain amount of hours and a reasonable break in between, although a lot of companies will expect you to sign a waiver for those rights. I was one week on one week off for about 6 months! That was pretty crap.
Find out the ins and outs before making a decision, some are good and some are awful!