Operate based on the idea that plant roots can grow while submerged in water as long as that water is extremely oxygenated.
The amount of water that is needed in a Hydroponic Deep Water Culture System will vary based on the size of the root system. Just starting out, when the plant is young and has a small root system, the water level should come up about 1″ from the bottom of the net pot(s) in the system.
Once the roots begin to grow down into the oxygenated water that has had fertiliser added to it, the water level should be lowered about 1-2″ a week until the bucket or container is between half and two-thirds full. The water level should stay at half to two-thirds full for the remainder of the growing cycle.