It is quite normal for a shipment to undergo customs clearance for around 24 hours, however if it has been there longer this could indicate that there are further details or paperwork required.

We would always recommend that you call DHL at the destination country to query what is needed to help clear it as that is the most direct way, and the couriers can provide you with a channel in which to provide any details/documentation. 

As customs are a separate governmental body they will not provide a timeframe for release to us or the couriers so we cannot advise on how long your shipment could be held by customs, however in most cases when you submit further paperwork to assist with the clearance process your shipment should hopefully clearly shortly thereafter.