In granting or modifying any permit, the commission may impose conditions or limitations on the proposed activity designed to carry out the public policy set forth in this chapter. Permits shall be valid for not more than five (5) years from the date of issuance, except that permits issued to a governmental entity shall be valid for not more than ten (10) years. The commission may issue one (1) extension not to exceed two (2) years in length if the applicant requests it in writing before the expiration date of the permit.