I'd say aproxamately a half a gallon (two liters) for that size planter. Water slowly so that is soaks into the soil evenly. Not much water should come out the drain holes. Check by feeling the weight of the planter. A nice healthy flowering plant (four weeks into flowering) should drink two liters in two days but a lot has to do with light intensity, heat and humidity. By the time it's finished flowering it may want twice that much water. Your plant at five nodes is not very thirsty yet. So two liters may last 3-4 days.

Let us know how that works.