19 CFR 12.108: Detention of articles; time in which to comply.

If the importer cannot produce the certificate or evidence required in § 12.107 at the time of making entry, the port director shall take the sculpture or mural into Customs custody and send it to a bonded warehouse or public store to be held at the risk and expense of the consignee until the certificate or evidence is presented to such officer. The certificate or evidence must be presented within 90 days after the date on which the sculpture or mural is taken into Customs custody, or such longer period as may be allowed by the port director for good cause shown.