The Shipper creation timing will depend on the following:
The Customer Request date on the Order | Schedule.
It will then look at the Picking days as defined in Order Management Setup | Other tab.
Then it will take into account the estimated number of days to ship as defined in the Ship Via used on the Order.
So if the pick days is 1 and the estimated shipping days is 3, the shipper will be created 4 days prior to the requested day.