29 C.F.R. § 20.215
The Department may sue any employer for any amount that the employer fails to withhold from wages owed and payable to an employee in accordance with §§ 20.207 and 20.209 . However, a suit may not be filed before the termination of the collection action involving a particular debtor, unless earlier filing is necessary to avoid expiration of any applicable statute of limitations period. For purposes of this subpart, "termination of the collection action" occurs when the agency has terminated collection action in accordance with the FCCS or other applicable standards. In any event, termination of the collection action will have been deemed to occur if the agency has not received any payments to satisfy the debt from the particular debtor whose wages were subject to garnishment, in whole or in part, for a period of 1 year.
29 C.F.R. §20.215