How much period can be covered in service tax scn

The service tax department conducted enquiry on assessee (Partnership Firm) on 02-01-2016; and find huge irregularity in service tax; the department wants to invoke extended period in terms of proviso to section 73 (1). How much period can be covered of FY 2010-11 (Ist half or IInd half or both) where assessee has filed its service tax returns for first half on 25-10-2010 and for second half on 25-04-2011 and deposited service tax liability on 5th of the following month from end of Quarter.