Many species
Cacti and most succulents want a thorough soak and then a long dry spell. Growth slows in winter, and watering should slow with it.
Short answer: for cactus and succulents, with the starting settings below (a 4 inch pot with a drainage hole), check the soil about every 5 to 7 days in spring and summer and every 7 to 10 days in winter, and water when the soil is dry all the way through. Give it a full drink until water runs out the bottom.
Your answer
Water when the soil reaches this point
How it shifts through the year
Water slowly until water runs out the drainage holes, then empty the saucer within an hour. The right amount depends on pot size, so go by drainage and how dry the soil is, not a fixed measure. Soak thoroughly, then let the soil dry out completely, and water less in winter.
Often every two to four weeks in warm, bright months and much less in winter. Check that the soil is fully dry first.
Many need much less, because they are growing slowly. Water sparingly, and only once the soil is completely dry.
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Related guides
Estimates use rules of thumb. Always confirm with your finger: push it about an inch into the soil before you water.