HOW many hours does it take to work a 40-hour week? That question is not as foolish as it sounds. For countless Americans who are paid by the week rather than the hour, a 40-hour week often stretches into 50, 60, even 70 hours. Driven by loyalty, ambition, or fear, they toil into the evening, putting work ahead of family life, social life, or solitude.But what happens when a conscientious employee who also happens to be a conscientious parent finally decides to say "Enough"? For one woman in Massachusetts - a single mother with a young child to care for - that decision cost her a job. Her managerial position at a high-tech firm began at 8:30 a.m. She never took a lunch hour. But when she left at 6:30 or 7 p.m. to pick up her child at a sitter's house, she says in an interview, "Eyebrows were raised around me." In time, even 10-hour days were not enough. Because of a pending merger, executives decreed that everyone would have to work even longer hours every evening, plus Saturdays, indefinitely. The woman quietly explained that she could not stay at the office that late because of her child, but that she would be happy to take projects home. The company refused. Two weeks later she was let go, with no definitive reason and no severance pay. "I feel I was wrongly discharged from that company," says the woman, who holds a master's degree and has always had an excellent employment record. "Single parents don't fit into corporate America. I don't know what we're going to do." Single fathers, too, can find their careers short-circuited by family obligations. This same woman's brother-in-law in Florida has sole responsibility for his three small children. Because their day-care center closes at 6 p.m., he cannot work late. When he was passed over for a promotion recently, he says his boss explained by saying, "Well, you're out of here by 5:30. You don't deserve a promotion." Most employees willingly put in extra hours when seasonal demands or special projects require it. But when excessive overtime becomes enshrined as a corporate way of life, does a salaried worker have a right to refuse? And even without parental responsibilities, is a 60- or 70-hour workweek necessary, or productive? Juliet B. Schor, an associate professor of economics at Harvard and author of a forthcoming book, "The Overworked American," calculates that in the past 20 years, the average work year of 1,949 hours has increased by 163 hours, making it nearly a month longer than it was in 1969. At the same time, working hours have decreased in the rest of the Western world. Ms. Schor cites a poll showing that Americans average only 16 1/2 hours of leisure a week. During the past decade, many businesses have made laudable progress in accommodating the needs of working families. But a widespread feeling persists that if parents can just find the right parental-leave benefits, the right child-care centers, the right after-school programs, they will be free to pursue their careers largely unencumbered by family demands. The out-of-work single mother disagrees. "People are combing the streets looking for longer day-care hours," she says. "But we need more reasonable work hours. The workplace needs to be totally restructured. There are all sorts of family needs that aren't being met. Many children are just not being cared for adequately. There has to be reform." Reform can include everything from flextime and part time to job sharing and telecommuting. In addition to giving families more time together, these options reduce what this woman terms "the exhaustion factor" and "the big word - guilt." Flexible work schedules also moderate what some employers call the "three o'clock syndrome," the rising anxiety and falling productivity that occur when working parents start worrying about their latchkey children after school. As long as employers continue to equate achievement with long hours, parents and non-parents alike will be denied an essential ingredient of a balanced life: a daily period of renewal and refreshment involving family, friends, books, music, sports - all the enhancements that make for a better human being, and in fact a better worker.