For germination and unrooted clones the amount of nutrient solution water a hydroponic plant needs each day is about 1/5 of a gallon or a bit less.
For seedlings and rooted clones the amount of nutrient solution water a hydroponic plant should get each day is about 1/4 to 1/2 of a gallon.
For early vegetative growth the amount of nutrient solution water a hydroponic plant should get each day is about 1/2 to 1 gallon.
For later vegetative growth and pre-flowering the amount of nutrient solution water a hydroponic plant should get each day is about 1 to 2 gallons for a plant up to 4 feet/1.2 meters in diameter. A larger plant needs more water.
For flowering the amount of nutrient solution water a hydroponic plant should get each day is about 1 to 3 gallons for plants with a canopy of up to 4 feet/1.2 meters in diameter. A larger plant needs more water.
Something else to take into consideration is the further the temperature rises over about 76°F/24.4°C, or the humidity falls below 60%, the more the plants water needs increase.
The amounts stated above have worked for the author, and will give you a rough idea of how often to discard and put new nutrient solution in a recirculating system or adjust the flow rate in a drain to waste system.
If you are currently growing a good way to tell if you are providing enough water is, to increase how often you change the nutrient solution with a recirculating system, or increase the flow rate with a drain to waste system.
Try it for a week or two, and if the plants do better with more frequent nutrient solution changes or an increased flow rate, you might want to maintain the increase in the amount of water the plants get.
With a recirculating hydroponic garden the nutrient solution will be supplied to plant roots, usually via pump, then the pump will turn off and the nutrient solution will flow back into the reservoir.
As water is absorbed by the plants or evaporates, your water reservoir level will drop. When it does, it is a good idea to replace it on a regular basis.
It is usually better to add water with no nutrients when you top up the reservoir tank for a recirculating, or deep water culture, hydroponic system. This is something you can experiment with.
With a drain to waste hydroponic system, nutrient solution is supplied to the plant roots then rather than flowing back into the reservoir, it is discarded.
As water evaporates and is delivered to the plants, your water reservoir level will drop. When it does, you will have to replace it. With a drain to waste system, add nutrient solution at full strength when you top up the reservoir tank.
Always measure, and if necessary adjust, the pH and strength of the nutrient solution after topping up the reservoir in either a recirculating or drain to waste hydroponic system.
If you have a recirculating or dwc hydroponic system, you may notice that after you change the nutrients, the pH rises. Then a few days after the nutrients were changed, you see the pH will start to fall.
When you observe this kind of pH rising after changing nutrients, then the pH lowering a few days later, it is a good indication that you are not changing the nutrient solution often enough.
Start changing the nutrient solution more often, so the nutrient solution is discarded and replaced before the pH starts to decline, or just when it starts to decline. Your plants will grow faster and be healthier.
Part 01) See the full hydroponic marijuana nutrient article here.
Part 02) Nutrients and hydroponic marijuana introduction here.
Part 03) Water and hydroponic nutrients here.
Part 04) Preparing hydroponic nutrients here.
Part 05) Measuring hydroponic nutrient solution - EC, TDS, PPM here.
Part 06) Hydroponic nutrient strength here.
Part 07) Hydroponic nutrient solution temperature here.
Part 08) Flushing or leeching hydroponic nutrient here.
Part 09) Amount of water for hydroponic marijuana plants here.
Part 10) Hydroponic nutrient list of materials here.