Del. Code tit. 30, § 543

If the Director fails to mail written notice of the Director’s action on any claim for credit or refund, or on any protest under § 523 of this title of a disallowance of a claim for credit or refund, within 6 months after such claim or protest was filed, the taxpayer may thereafter, and prior to notice of action by the Director, consider such claim or protest to have been disallowed by the Director, and the taxpayer may, at any time prior to the expiration of 3 years from the date on which such claim or protest was filed, proceed pursuant to § 542 or § 544 of this title, as the case may be.