1. ask your courier for relevant Customs phone number, and Custom working reference.
2. Call them by youself and let them explain why they seize it and when they will release it. Show your cooperation with their investigation.
3. Let your courier keep you informed whenever they got it released from Customs.
4. Write them if necessary.
Theoratically Customs can take as long time as they wish for investigation, with regret. However, the courier is responsible for a request on releasing this on-hold. Your mail express service is supposed to deal with this on your behalf. The early they write Customs, the early you get the answer.
When you rang a given phone number /left an msg without answer within 24 hours, you can call the person who refered you that number and mention this is your recall bla bla. It works exactly the same way as in China.