At one point, you may receive a letter from the IRS informing you of an audit on your Illinois business. This can be unnerving, but it does not have to be if you prepare for it. One thing to keep in mind, according to H&R Block, is the process can be lengthy depending on the method by which the IRS conducts the audit. The agent may do your audit by mail, at a local IRS office or in person. In person audits typically take the longest.