as corriene says, it depends on your job setting. Many OTs inn hospitals, nursing homes and schools work a 40 hour week but many of these same places hire contract or part time staff; you can say how much you want to work but the facility may dictate the hours. At some nursing homes/rehab centers it depends on the patient case load; when the work is done, you go home no matter how many hours. I am a contractor in home health OT and I have complete control of my schedule - I can accept as many patients as I want to and schedule them as I (and the patients) want, no control from the agency.
Working as an OT really depends on the job setting and how many hours you want to get paid for. As a contractor you have more flexibility to choose schedule , but again it depends on the needs of the setting ...........most I know work more than 40 hours to make more money.