Most SCUBA tanks are made out of aluminum. Aluminum comes with its own set of advantages and disadvantages. As for the latter, aluminum tanks, being lighter, are typically more buoyant, meaning the diver must wear extra weights to prevent being pulled by positive buoyancy to the surface. On the other hand, when the diver is not in the water, carrying around aluminum tanks is a lot easier than lugging steel ones. Aluminum tanks are also less susceptible to rust than steel tanks. Aluminum tanks last around 15 years, and cost about half of what steel tanks do.
If your tank isn't aluminum, it's probably made of steel. A steel tank's heavier weight actually works to a diver's advantage, since no extra belt weights are typically required. The tank weighs the diver down all on its own. Out of the water, though, the weight of a steel tank can be burdensome. Steel tanks are more susceptible to rust than aluminum tanks, thus requiring more in terms of maintenance. Steel tanks last around 30 years. They are typically around twice as expensive as aluminum tanks.
Whether your tank is steel or aluminum, it will generally come in one of three sizes--or capacities. Low-capacity tanks hold 2400 to 2640 psi (pounds per square inch). Though many factors--like workload, depth, and temperature--go into how long a tank will last underwater, a low-capacity tank on a typical 60-foot dive will generally last around 45 minutes.
A standard-capacity SCUBA tank holds around 3000 psi. This translates, generally speaking, to about an hour underwater on a 60-foot dive.
A high-capacity SCUBA tank holds between 3300 and 3500 psi, or enough air to last approximately 80 minutes or so at a depth of around 60 feet. Again, how hard you have to work, the temperature of the water, and the depth of the dive can greatly affect these times.