Exactly what do you need to know? The time it takes to convert a purchase requisition line into a purchase order? Is that the time it takes from when you press the button to create the orders until they are created, or??
As default document date is always set to the same as the order date, but allow you to manually change it. The document date is not a match toward the order date, but the posting date. It is primary used if you for example receives the purchase invoice on the 4th of the month. But the date your vendor used as invoice date was the 30th of the previous month. And you company have already “closed” this month. So in order to still being able to register it “correctly” then you use the 4th. as your posting date, but enters the 30th. as the document date. Changing the document date will then update the due date accordingly.