In one experiment presented by the California Energy Commission, two D-sized Duracell batteries were able to power an ordinary flashlight for 15 hours. Had the batteries been powering a less powerful or more efficient flashlight, they would have lasted longer than 15 hours.
The length of time that a Duracell battery lasts varies based on two main factors. The first factor is the size of the battery. The second factor is the amount of energy that is drawn from the battery. Thus, a small Duracell battery generating the same amount of energy as a large Duracell battery burns out faster.