If the refund is agisnt export, it should be filed within six months of the end of the quarter in which export is effected. Refer to notification No. 41/2007 as ammended by notification No. 32/2008 of service tax in this regard.
If the same is for export of services either a refund under Rule 5 of the CCR or a rebate under Not.11 & 12 opf 2005 then the time limit is 1 year from the date of export.