Calathea and Maranta
Calatheas and prayer plants like consistently damp soil and are often sensitive to minerals in tap water. Leaves that curl are their way of asking for a change.
Short answer: for calathea and prayer plants, with the starting settings below (a 6 inch pot with a drainage hole), check the soil about every 5 to 7 days in spring and summer and every 7 to 11 days in winter, and water when the top half inch of soil is just drying out. 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
Check the soil about every 5 to 7 days in spring and summer and water only when the top half inch of soil is just drying out. In winter, growth slows and soil stays wet longer, so expect roughly 7 to 11 days between checks. Always confirm with your finger first.
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. Many are sensitive to minerals in tap water, so use room temperature water, and filtered or rainwater if leaf edges brown.
Curling usually means thirst or dry air. Check the soil, and move the plant away from heating vents if the air is dry.
Many owners get better results with filtered or rainwater, since these plants can be sensitive to minerals in hard tap water.
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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.