Optimum times for Ebb & Flow system?

[GregoryJ]

Very new to hydroponics - I made the 11 bottle system and it works well. Tomatoes are growing. I've read that most people use the 15 min on/off timers, but is there an optimum time for soaking the roots and then draining? Maybe flood for 5 minutes and drain for 20? any thoughts are appreciated. Thanks!