An S corporation would never file an 1120, but it would file an 1120S by March 15 every year that it exists, even if it had no activity during a tax year.An S Corporation would file a 940 by Jan 31 every year that it had payroll in the prior calendar year or if it filed a 941 for the prior year.An S Corporation would file a 941 every quarter once it starts paying payroll. Given that S corporations are required to pay their active shareholders reasonable wages, that should be every year that the S Corporation has any business activity.