My drips are full open and run 24/7. Lava rock or grow rocks don't hold enough water to last more then 4 or 5 hours. It will dry out quickly and then the plants will start to use up stored energy in the leaves. Ebb & flow type of system are different. 15 on 90 off is not bad, but it depends on the temperature and how fast the grow rocks dry out on the bottom.