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.
Building to a Higher Standard. When I was a builder, I built residential decks based on a 70-psf design load (60-psf live load, plus 10-psf dead load). For a 12-foot span (beam at the edge of the deck), our base-case example of 2x10s at 24 inches on-center does not meet that standard.