What's the time frame in which doctors must refund any overpayment to patients?

A year and a half ago my husband went to a doctor, had surgery, and the medical problem was over. No follow-up appointments scheduled. Last week, he goes back to the same doctor for an unrelated issue and is told he has a “little credit” there so we don’t have to pay the co-pay today ($20). I asked what the credit was, and was then told $233. Is it legal for them to have held our money for so long?! What if we’d never gone back to that same doctor? I would have never known about the overpayment! The lady in patient accounts didn’t really have an answer for me. Now they won’t submit the check until these charges come through from the insurance company which I can understand, but I’m still upset that our money has been held for so long. Is this legal and can I file a complaint? It makes me want to call every doctor I’ve ever been to and ask if we have a credit there and for them to send a check!