Answer
15 hours.
Work Step by Step
Assume the temperature remains constant. The gas will stop flowing when the final pressure inside the cylinder is atmospheric pressure. Find the volume that the original amount of gas takes up at atmospheric pressure.
$$P_1V_1=P_2V_2$$
$$V_2=V_1\frac{P_1}{P_2}=(14L)\frac{1.38\times10^7 Pa+1.013\times10^5 Pa}{1.013\times10^5 Pa }=1921L$$
14L will remain in the cylinder at the end, so 1907 L flows out. At a rate of 2.1L/minute, it will take (1907/2.1) = 908 minutes to finish, or about 15 hours.
