How often green plants need water?

As most plants seems to be happy with once-a-week watering rhythm, but truly, there is no calendar for watering. It all depends of the light, temperature, air humidity, time of the year, plant type and size, pot size and material, dusty level on the leaves, nutrition levels, type of the potting medium/soil...

Just stick the finger in the soil and check little bit deeper if the soil is still moist or drying out. There might still be enough moist after top part 2-3cm is dried out. Bigger the pot, more moist underneath. Plants need normally more at the summer time (growing period) and less at winter.

Keep in mind that house plant roots need also air between watering. When you water, water thoroughly! After watering, let it drip a while before putting back to ceramic pot. Some of the plants need longer aerial periods between watering than others. Don´t water "just in case" or if week has passed since last watering but soil is still moist.

There are of course some swamp plants and Papyrus plants whose roots should always be completely wet and desert plants which sand should dry out completely for months. Normal house plants are something between these two extremes.