According to the Factories Act, the provision is laid down under the section 51, 54, 55, & 56.
Although the maximum working hours is 9 hours, but it can be spread over to 12 hours per day.
12 hours is for people who said to be working under the "Compressed Shift". Shift rotation that is managed accordingly scheduling the no. of working days and allowing the employees corresponding to take offs is called compressed shift.
Under the provision of Factories Act, you can refer the following:
Section 51. WEEKLY HOURS. - No adult workers shall be required or allowed to work in a factory for more than forty-eight hours in any week.
Section 54. DAILY HOURS. - Subject to the provisions of section 51, no adult worker shall be required or allowed to work in a factory for more than nine hours in any day: Provided that, subject to the previous approval of the Chief inspector, the daily maximum hours specified in this section may be exceeded in order to facilitate the change of shifts. [B
Section 55. INTERVALS FOR REST. - (1) The periods of work of adult workers in a factory each day shall be so fixed that no period shall exceed five hours and that no worker shall work for more than five hours before he has had an interval for rest of at least half an hour.
(2) The State Government or, subject to the control of the State Government, the Chief Inspector, may, by written order and for the reasons specified therein, exempt any factory from the provisions of sub-section (1) so however that the total number of hours worked by a worker without an interval does not exceed six.
Section 56. SPREADOVER. - The periods of work of an adult worker in a factory shall be so arranged that inclusive of his intervals for rest under section 55, they shall not spread over more than ten and a half hours in any day. Provided that the Chief Inspector may, for reasons to be specified in writing, increase the spread over up to twelve hours.