The dealer who claims refund due to sale effected by him under sub-section (1) of section 18 shall file an application in Form W to the assessing authority along with copies of invoices or bills of related purchases within one hundred and eighty days from the date of accrual of such claim. The assessing authority after verification of the correctness of the claim, shall issue refund within ninety days from the date of the receipt of the application in Form W.
For more info: - http://tnvat.gov.in/English/VATRules_290107.pdf