It's hard to tell you because there are so many variables that come into play: your home's environment, where it's growing, the pot size, the composition of the soil, and the time of year. You'll probably have to water more in the summer months and less in the winter months. Yes, you can overwater any plant. Even though a Peace Lily plant likes slightly moist soil, you can kill it by watering too often or keeping the saucer full of water.