No and yes. Employers can set any length of workweek; the FLSA doesn't enforce a legal minimum or maximum number of hours. But if an employee is nonexempt, her boss must pay overtime past 40 hours. If she is exempt, however, an employer can require the additional hours at a set salary - and dock her pay if she doesn't comply. But her check can't dip below the $455 a week threshold without potentially invalidating her exemption.