Do you know why nobody has answered your post ? Because your question is like asking how long a piece of string is. Heres a simple way to know how much water your plants need at any one time : Fill the buckets with your prepared soil and water heavily until the soil is saturated and some water runs out the drain holes. Leave standing for 24 hours so that the soil biological processes get up and running. Plant yr. plants. If they're seedlings or small plants they won't need any water for weeks in such a big pot. I recommend you then get a soil moisture measurement meter. Its got two prongs and a meter readout. Stick it in the soil and it tells you how moist it is. Simple. Tips for watering soil : Make sure you break up the topmost layer of soil at regular intervals this helps greatly in avoiding dry spots. Dry spots are pockets of dry soil in your buckets. How much water you will eventually use depends on a mulitiude of things like temperature, humidity, nature of soil, size of plants, etc, etc. Good Luck.