Del. Code tit. 9, § 4634

The last installment of each authorized issue of bonds shall be paid not later than the date of expiration of the period of usefulness of the project for the financing of which such bonds are issued, as determined in the bond resolution authorizing the issuance of the bonds, computed from the date of issuance of the bonds or of the issuance of the first note or notes, whichever is the earlier.