Ill. Admin. Code tit. 35, § 734.665 – Audits and Access to Records; Records Retention

a) Owners or operators that submit a report, plan, budget, application for payment, or any other data or document under this Part must maintain all books, records, documents, and other evidence directly pertinent to the report, plan, budget, application for payment, data, or document, including but not limited to all financial information and data used in the preparation or support of applications for payment. All books, records, documents, and other evidence must be maintained in accordance with accepted business practices and appropriate accounting procedures and practices.
b) The Agency or any of its duly authorized representatives must have access to the books, records, documents, and other evidence set forth in subsection (a) of this Section during normal business hours for the purpose of inspection, audit, and copying. Owners or operators must provide proper facilities for such access and inspection.
c) Owners or operators must maintain the books, records, documents, and other evidence set forth in subsection (a) of this Section and make them available to the Agency or its authorized representative until the latest of the following:
1. 1) The expiration of 4 years after the date the Agency issues a No Further Remediation Letter pursuant to Subpart G of this Part;
2. 2) For books, records, documents, or other evidence relating to an appeal, litigation, or other dispute or claim, the expiration of 3 years after the date of the final disposition of the appeal, litigation, or other dispute or claim; or
3) The expiration of any other applicable record retention period.