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