How to determine flood and drain timings?

If using a flood and drain system that recirculates the nutes, using clay pebbles and the buckets are 10L outer bucket and 6L inner pot, how would you figure out the timings for flood and drain?
Do they vary during the life of the plant or between veg and flower? Does it depend on strain? How do people figure out how to set the timings?
What is the theory?
Submerge the roots long enough to uptake some nutes and then keep the roots in air as long as possible before drying out?

My set up is run for 15 minutes(cheap timer)1 time to 2 times a day.
Using 2 gallon flood pots and mini rock wool cubes
3x6 tray and 3x3 tray 35 gallon res...alternate the pumps,so the res will not empty out.
Small res saves on nutes here

I run mine 3x per day, I don't run at night.

Shortly after the lights come on it floods, then drains. Then again 4hrs after the drain is finished, and again before/at lights off.

Has anyone ever run a water farm?