No time limit. According to the "Regulations of the People's Republic of China on the Customs Protection of Intellectual Property Rights" and the corresponding implementation measures, if the situation you described involves a trademark that has been registered for protection at the General Administration of Customs, the customs will initiate protection procedures ex officio, and there will be no 20 tasks day limit. However, if the customs cannot determine that the goods are infringing after investigation and do not receive a court seizure request within 50 working days from the date of detaining the goods, the goods shall be released.