An ebb and flow system floods the grow tray with nutrient solution and then drains it back to the reservoir. A timer controls the pump, so the timing matters.
How long to flood
Flood long enough for the medium to soak up solution, then stop. Medium that holds water well needs less time than medium that drains quickly. Adjust in small steps and watch the plants.
Drain fully
The tray should drain almost completely between floods. Standing water can keep roots too wet and low in oxygen.
Adjust with the plants
As plants grow, they drink more, so the cycle may need changing. Check the medium and the roots regularly.