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.
Most of the 24' spans I see use 2x4 bottom chord-I'd up that to a 2x6 for insurance. Most also use 3 each 8' 2x4 for the bottom-I'd use 2 x 6 x 12 with a single splice plus the gussets at the angles. I'll bet if you tried to buy a 2 x 4 x 24 these days it'd look more like a corkscrew than a board.