As with Yellowstone National Park in Wyoming, geothermal energy can be harnessed for thousands of years before it runs out. Iceland is another example of abundant geothermal energy. Magma pockets below the surface of the Earth will move on eventually--that's how the Hawaiian Islands were formed. However, for now, geothermal pockets will be stable for thousands of years.
No, geothermal energy is not a consistent and reliable form of energy, because its productivity depends on the mercy of mother Earth. This is not to say that geothermal energy is not good and desirable. However, as a form of energy alone, if probably needs supplementation from other forms of energy, such as fossil fuels. Standing alone, geothermal is risky, but it can be a positive thing with other types of energy.