There was a question in the About.com Job Search Discussion Forum yesterday from someone who is tired of working overtime. She said that "These additional duties have required me to work 6 days a week for at least 3-4 weeks of every month for the last 3 years." I'd be tired, too.
Unfortunately, unless she is under 16, works in a state that limits required overtime for some employees, or is covered by an employment contract or collective bargaining agreement, her employer can require mandatory overtime.
Another question that comes up frequently is whether an employer has to pay you overtime pay. For non-exempt employees the answer is yes and the amount you must be paid is not less than time and half. In some cases, exempt employees must also be paid overtime, depending on their job. Here's more on overtime pay and when and how much you will get paid.