Usually, once every other day is more than enough for most cannabis plants. Give them some time to slightly dry out, but not to the point of withering, of course. Here’s a good way of finding out if your plant is thirsty: dip a finger into the soil up to the point where your first knuckle is covered. That’ll be about 5cm. Dry soil means it needs watering, while damp soil means you can skip for another day.