It's a state by state law. You want to research Recoup / Recovery time limits for your state. Some have no limit, some are 6 months, some are 2 years. Not likely the carrier violated that time limit, all the carriers have pretty tough edits on their system to account for state/federal laws.
I should also note that it depends on the claim and what's being recouped, if there was an error in eligibility the carrier has a right to recoup any/all payments because they have a right to carry out an investigation. If there are claims paid in error involving dollar amount, most likely the carrier has the edits by state law loaded so not likely they are in the wrong, if you don't send them the money they may have the right to deduct from future claims.