I have 20 cacti and i am not used to the watering cycle...i dont know when to water them.... is once a moth good or more?
The frequency will depend on the size of the pot, the type of cactus and your environment. Don't water cactus until the surface of the soil is dry. They will often go a month without water when they are well established, but if you have young plants or recently repotted they may need more. Never allow the pot to soak or sit in water. Also avoid getting water on the plants, just water the soil.
In addition, many desert cacti need a period in winter when they never get watered. For desert cacti the safest bet is to let the soil go totally dry between waterings. Fear not, they can take it. This could only take a day in the middle of summer if you have it outside or longer if it is late fall and you have it inside.
I agree, just letting the surface go dry (vs most of the soil in the pot) is going to end up much too wet for them. You could leave them for more than 2 months in winter without a problem most of the time. You should also learn to see very subtle changes in them too though, so you gradually get to know when they may need some, and when they don't. The temptation to water plants is very strong, but we need to resist it a lot of the time because growing them indoors, with less light than they'd get outside, in small pots with only a small drain hole, and using 'potting soil' that's usually full of peat that doesn't drain well at all but keeps everything soggy, is a real challenge.
