If you accept a new job, only to discover that your responsibilities aren't what the boss said they would be, you should speak up. However, there is a question as to how much time should elapse before you raise your concern over the job. Even in the most disappointing jobs, patience can be a virtue; some analysts say ninety days is a fair amount of time to wait before speaking up. There are several ways of dealing with the situation, some better than others.
See "The Workplace: When a job isn't what you expected", Matt Villano, International Herald Tribune, November 15, 2005