Generally speaking, federal law is silent regarding the number of hours an employer may require an adult worker to work in a day except in specific circumstances. State law is also mum on this issue, although California does have a law that speaks to it indirectly. Both federal and state laws limit the hours that may be required of younger workers.
The Federal Fair Labor Standards Act
The Federal Fair Labor Standards Act of 1938 covers most workers, although some are exempt from its provisions. For non-exempt workers, the U.S. Department of Labor's interpretation of the law regarding maximum working hours for adults is clear: FLSA does not limit the number of hours in a day or days in a week an employee may be required or scheduled to work, including overtime hours, if the employee is at least 16 years old.
The FSLA also is silent on the issue of required overtime. However, employers generally have the right under federal law to fire an employee who refuses overtime work.
State Laws Governing Limitation of Hours Worked
A few states have rules that tend to limit most adult employee's working hours, but none has an absolute prohibition against it. Even in states with laws that treat the issue, the limitations on hours worked are indirect and weak. In California, for example, an employee cannot be fired or disciplined for refusing to work more than 72 hours in a given week. New York labor labor requires employers to pay employees working more than 10 hours a day one extra hour of pay, but at the minimum legal wage.
Otherwise, as one labor lawyer commented, no one may work more than 168 hours per week, but only because physics prevents it -- there are only 168 hours in a week, after all.
Laws Protecting Younger Workers
The FLSA provides that workers under 16 years of age cannot work more than 8 hours a day, with a lower limit of 3 hours daily on school days. Some states have slightly less restrictive limits. Idaho, for instance, limits workers under 16 to nine hour working days, with no further reduction for school days.
Although the FLSA normally prevails over state laws, small businesses are exempt from its provisions. Further, if the employer is not engaged in interstate commerce, only the less restrictive state laws apply. In practice, this can be a somewhat tricky distinction. An office worker who mails statements to local customers is not covered, but an office worker who prepares promotional material subsequently mailed to individuals in other states probably is.
Certain workers are exempt from the provisions of the FLSA and are covered by other laws and regulations limiting hours worked. Unionized workers are protected by labor agreements with employers. Other workers are covered by safety regulations. These include airline pilots, physicians, truck drivers and miners working underground.