There have been three parameters that SAP will take it into calculation to determine the delivery date and release date during material requirement planning. They are -
1) Planned Delivery Time which is the vendor-dependent lead time being measured from the time PO is outputted (for sending to the vendor) till the time the shipment is delivered at the warehouse.
2) GR processing time represents the necessary period of time being required for receipting the shipment into the warehouse. This period is, usually, for quality and quantity inspection.
3) Purchasing Processing Time. This is the internal required time for processing the purchasing document (Purchase Requisition) till Purchase Order. You can set it up plant-wise via Transaction OMI8.
With these three parameters, it is possible that you can set it up correctly in your system to enable the appropriate planning of your material requirement.