Just like any other cacti you have to water them in spring always after 7 sunny days. If the cactus is bigger than 15-20 cm, it is enough to water it every 2-3 weeks. Very big plants can do without water for many weeks without any harm. Bigger plants need bigger pots with more soil, therefore they dry up slower. You have to make sure that if the plants are kept cool during winter, their soil must have been completly dried up. It is possible to keep them at room temperature during the whole year, though you ought to water them a bit in winter as well. It is better just to shower them.