The University of Arizona recommends watering agave plants every two weeks during the hot weather (summer and autumn) and watering monthly in temperate times of year. Agave goes dormant in the winter, so too much water during this time can cause the plant to rot.
Add water to your agave plant until you see liquid flow out the drainage holes on the bottom of the container, or until the soil around an in-ground plant becomes saturated with water. The exact quantity of water needed varies by the size of the plant; larger plants have bigger roots, so they take more water.
If you have outdoor agave (either in the ground or in containers), be aware of the effect of rainfall. Skip your watering if the plants receive rain so you don't give them too much water.