Do you want to enjoy your plants for a long time? If you look regularly at the plant to see how it feels, you can estimate the need of the plant. You do this by looking to the soil, the leaves and potential flowers of the plant.
Next to this every plant has its favorite spot. At the label in the plant and this website, you can learn more per product about this spot.
Every plant uses its received water to suck it in for transportation to the leaves and potential flower stems. This is what makes the plant grow and flower.
The moisture evaporates by this process, what improves the humidity in the area of the plant. The more square centimetres of green surface the plants contains, the better the air will be purified by the plant. The moisture that the plant evaporates, is the amount of water that you need to complement.