They remain in "Prepared for Shipping" status until all pallets are ready to be picked up. The "system" operates 24/7.
I order lots of computers and other Apple gear out of China. The only long delay I've experienced is when one shipment arrived at the customs station in Alaska. There was an error in the paperwork causing an eight day delay while Apple and Customs sorted it out.
That aside, I've found that Apple and FedEx do a fantastic job of providing swift service out of China.