Is there a limit to how much interest you can charge to an outstanding balance?

If your contract permits you to charge interest in excess of 10%/year on unpaid sums, then you may do so. If not, the Mechanics Lien Act follows the lead of the Prompt Payment Act, which provides as follows:

• Payment on private construction projects in Illinois is regulated by §815 ILCS 603/1 et seq. (the "Prompt Payment Act"), which applies to all private projects for more than 12 residential units.
• Once the GC has performed in accordance with the contract, it is entitled to payment from the owner within 15 calendar days. The owner must approve payment within 25 days.
• If the owner disputes the request, they must send notice within 25 days, or the request is deemed approved. Failure to send notice, or otherwise late or wrongfully withheld payments will be subject to interest at the rate of 10% per annum (0.833%/mo.).

I hope this informaiton was helpful. Best of luck!