How long mail takes is going to depend on many factors along the way. Many factors, indeed, though with regard to how long does priority mail take, the answer is, not long. In fact, the United States Postal Service says your package or envelope will arrive within two days.
Both the 2009 IRC and the DCA6-09 allow #2 southern pine 2x10s to span 13 feet 1 inch at 24 inches on-center at a design load of 50 psf. That means these joists can span the 12-foot depth of the deck, and the beam can be located at the perimeter. Nine joists will be needed for the 16-foot width.
2x6 or 2x8 for deck joist. I strongly recommend you refer to code for your area. For example, my local code (IRC) specifies that the maximum allowable span for a 2x8 joist depends on the spacing of the joists (which you did not mention in your post), and the species of wood (which you did not mention in your post).