In an ebb and flow system you don't want the solution around the roots for too long, just enough to soak whatever medium you are using that the roots grow in. About 15 minutes to get all of the roots soaked and then start to drain. During your daylight cycle, maybe four times at 15 mins. each should be good with one right when the lights come on and the last one about a half hour before lights out as you don't want the roots sitting there soaking in the dark. You will have to judge by what your plants do. Keep a close eye on the ppm's and with rock wool, the ph to some degree. You will probably have to top water until the roots are established, usually a week. Add your nutes right away but start out on the light side, like 400/500 ppm and work your way up, depending on what solution you are using. Go for a week at weak solution, and add up from there over another weeks time, up to whatever your system is meant to have. Once again, it can be anywhere from 1000ppm to 1500ppm.