Calculating battery life of Arduino + XBee Project
The device is not sleeping 1.3% of the time. \$ 66 \times 0.013 + 28.5 \times 0.986 = 28.9mA\$ average load. \$\dfrac{780mAh}{28.9mA} =  27 \; hours\$, assuming you get every ounce of the 780mAh out of your battery.  You should probably derate that number by at least 20%.