The report must be filed with the Illinois Department of Transportation (DOT) no later than 10 days after the accident. If you don't file a report when required, then you may also face penalties such as fines, jail time, or having your driver's license suspended.
After being involved in a car accident, you should report it to your car insurance provider as soon as you can. Many insurers specify that you need to inform them about an accident within 24 hours of the incident.