If you want to grow your plant properly, keep the soil moist, but avoid over-watering. The ZZ plant likes a consistent moisture level of about 60-70%. You can get a good idea of this level by using a moisture meter. This tool measures the water content of the soil. A reading of 60% means that there’s only half as much water as there should be in the soil. It's not too dry, but not too wet either. When the reading reaches 80%, then you know the soil needs more water. It's important to note that soil will dry out during the day and then start to re-hydrate when it gets dark.