Seven years. Illinois has no statute overriding federal law (Fair Credit Reporting Act) which mandates that criminal background checks may only go back seven years. However a federal level background check for a high security clearance, state licensure, or MEPS can go back for as long as you have been alive.
Illinois state law:
Abide by the Federal Trade Commission’s Fair Credit Reporting Act (FCRA), 15 U. S. C. § 1661 et seq. and the following state ruling:
(20 ILCS 2635/7 (A) (1) (2) (from ch. 38, par. 1607) – Illinois Uniform Conviction Information Act
The following provisions shall apply to requests submitted pursuant to this Act for employment or licensing purposes or submitted to comply with the provisions of subsection (B) of this Section: (1) A requester shall, in the form and manner prescribed by the Department (Illinois Department of State Police), submit a request to the Department, and maintain on file for at least 2 years a release signed by the individual to whom the information request pertains. The Department shall furnish the requester with a copy of its response. (2) Each requester of conviction information furnished by the Department shall provide the individual named in the request with a copy of the response furnished by the Department. Within 7 working days of receipt of such copy, the individual shall have the obligation and responsibility to notify the requester if the information is inaccurate or incomplete.