I'd allow 20 days from payment, before I contacted the seller. Then ask him for the Delivery Confirmation number and date of shipping.
Don't get too worried about the DC number, it is for the seller's benefit and your parcel would have cost over $20 to ship with DC. Mostly you want to know when and how the parcel was sent.
On Day 20, if the parcel has not arrived, mark Day 40 on your calendar and start an Item Not Received dispute.
But mostly, relax, this is the postal system. It really is very reliable.