If the question that you ask refers to freshwater aquariums, then your tank can hold up to 100+ gallons of water, depending on what size you buy. In pet stores they commonly sell 10,15,and 20 gallon...
It all depends on the plant. You'll know you given your plant enough water when after watering your plant will not suck up any more and a small puddle appears on the soil still after waiting about a...
How much water cactuses hold depends upon thesize of the cactus. The total amount can figure quite high. About 90% of the cactus body is devoted to holding and storing water.