What an be done if an exempt salaried employee is expected to work unrealistic hours?

Unfortunately, there is no legal limit on the hours an exempt salaried employee can be required to work, outside of 1) any limitations contained in a written employment contract (if any), and 2) any profession/job-related limitations, which exist for a very few careers (e.g. airline pilots). Otherwise, given the concept of "employment at will," an exempt salaried employee can be made to work any number of hours--the employer has free discretion to set hours. The employee's recourse is to resign/quit and seek other employment, if he/she does not like the hours. 
Of course, just because an employee is *called* exempt does not mean he or she is--salaried employees can be non-exempt, if they don't qualify for the overtime exemption--and this puts a check on unreasonable hours, since the employer would have to pay overtime for work past 40 hours per week. To be exempt, a salaried employee must be paid now at least $47,000 (actually, it's slightly over $47,000) *and* meet one or more of the tests for exemption, such as the executive (which should be called managerial, since it applies to non-executive managers), administrative, or professional tests. These tests can be found on the U.S. Dept. of Labor website; the criteria under each test should be compared vs. the employee's job. If the employee either earns too little or does not qualify under at least one test, he or she must be paid overtime, despite earning a salary. If worked more than 40 hours without overtime, a non-exempt salaried employee could file a complaint with the department of labor and/or speak with an employment law attorney about suing.