Thanks, Nancy. For some reason I was thinking a year as well, or even 18 months but since my post I found this in the Medicare Hospital Manual:

415.11 DRG Changes.--Under prospective payment DRG bill adjustments are required from you where errors occur in diagnoses and procedure coding that change the DRG, or where the deductible or utilization is affected. You are allowed 60 days from the date of the intermediary payment notice for adjustment bills where diagnostic or procedure coding was in error. Adjustments reported by the PRO have no corresponding time limit and are adjusted automatically by the intermediary without requiring you to submit an adjustment bill. However, if diagnostic and procedure coding errors have no effect on the DRG, adjustment bills are not required. (See ยง414.1E.)

Perhaps outpatient or physician claims have a year?