No, there is no limit on how many hours a nurse can be made to work; the shifts you describe are legal, though the hospital must "pay" for it--that is, if you the nurse is not exempt from overtime, then he or she must be paid at overtime rates (time-and-a-half) for all hours worked over 40 in a workweek. Very few nurses--typically only those with significant managerial or administrative responsibilities--should be exempt from overtime (to see the tests for when someone is exempt from overtime, go to the federal Department of Labor website and look under "wages"); that means, that if you're working 60 hours a week (5 days x 12 hours/day), you should receiving 20 hours of overtime pay, over being paid for the equivalent of 70 hours pay that week. So the nurse can be made to work longer, but must be compensated for it.