When they need water. You can't really set a schedule cause there are too many factors. Like how big they are, some strains drink more than others, heat might cause more water to evaporate in some setups, what size pot you have them in etc. You want a good dry period inbetween waterings so the roots get lotsa air. Use the lift test, just pick them up and judge as best you can by weight. you'll develop a feel for what's proper after awhile. Like so many things in life, growing is part science part art and only experience will teach you everything you need to know.

Learn to recognize the first signs of underwater, the droopy somewhat limp fan leaves, and see how long it takes, then water 1 day before they get like that. New growers usually over do everything, nutes, water etc. More is not better in this case.