Originally Posted by flyindarkness
most people who are serious about the job usually spends 20 hours plus minimum.
Some people will "get it" with 20 live cases, some people will need 100 to be fully comfortable.
What firm if you don't mind me asking? If it's the top three, definitely take some time and practice, there are a lot of good resources on the net, no need to buy a book.
+1. If it's MBB, the case will make or break it, and the people against which you are competing will have spent quite a bit of time preparing for it.
If it's not MBB, it will depend on the firm, and on with which division within the firm you are interviewing. The classic strategy divisions of the big four will also heavily rely upon the case, but other divisions less so. Even there though, you'd do well to spend a bit of time prepping for a case study.