Mont. Code Ann. § 10-4-205

If the amount paid by a provider or seller to the department of revenue exceeds the amount of fee owed, the department of revenue shall refund the amount of the excess payment, with interest on the excess payment at the rate of 0.5% a month or fraction of a month from the date of payment of the excess until the date of the refund. A refund may not be made to a provider or seller who fails to claim the refund within 5 years after the due date for filing of the return with respect to which the claim for refund relates.