According to Techlib, battery life is calculated by dividing the capacity by the actual load current. The capacity of a 9-volt battery is 500 milliampere hours, and its load current is 15 milliamperes. Therefore, it has a 33-hour battery life.
Lower drain currents lead to longer battery life, and the 9-volt battery beats the AA, 6-volt lantern, D and C batteries in this category. In contrast, the AAA battery has the longest battery life at 100 hours. Techlib also notes that silver-zinc batteries have 85 percent of the capacity of ordinary alkaline batteries of the same size, while lead-acid batteries have 35 percent and nickel-cadmium batteries have 30 percent.