In the State of Illinois how long does an employer have to give you your last check if you've been fired?

You have to get the last check no later than when you would have been paid that money if you were still working for the company.  For example, if you get paid every two weeks on Friday, for the two weeks that ended one week ago, and you were fired on payday, you'd get your check for your last week of work no later than the next payday two weeks later.

If it doesn't happen, you can sue, or you can make your claim through the Illinois Department of Labor's Wage Claims section.