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.
Your answer
Water when the soil reaches this point
How it shifts through the year
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.