Spathiphyllum
Peace lilies prefer evenly moist soil and let you know loudly when they are thirsty by collapsing. They bounce back fast, but repeated wilting stresses the plant.
Short answer: for a peace lily, with the starting settings below (a 6 inch pot with a drainage hole), check the soil about every 6 to 9 days in spring and summer and every 9 to 14 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 6 to 9 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 9 to 14 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. Peace lilies like evenly moist soil, so water thoroughly each time and never leave the pot standing in water.
Most often it is thirst, and it perks up within hours of watering. If it droops while the soil is wet, the roots may be rotting.
They like even moisture, not constant soaking. Let the very top dry slightly, and make sure the pot drains.
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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.