Hi Sugan,
Implementation of 'Planning time fence' will delay procurement for ugent orders. In fact , it will go against your objective of delivering urgent customer order within week. Leve it 0.
Reason for using planning strategy 40 is to make for forecast as well as customer order. You should run MRP as soon as urgent customer order is entered & use pegging report(MD09) to identify Raw material shortages.
"Planned Delivery Time' is SAP field for business requirement 'Procurement lead time'(Business term). They are same.
Hope it helps,
Jatin