35 ILCS 505/15.1

The Department shall pay all refunds due under this Act within 90 days after receipt of application for a refund. For commercial motor vehicles, refunds paid after the expiration of the 90 day period shall bear interest at the rate of 1% per month or fraction of a month. Refunds paid to all other persons subject to this Act shall be paid at the rate and in the manner set by the Uniform Penalty and Interest Act.