Rock Wool is one of those mediums that is always wet. You can get away with watering 1x a day but the standard is either every 2hrs for 10min or every 3hrs for 15min. The strain I'm running right now can't handle that much watering. I've got it watering approximately every 4hrs for 15min then and hour before lights out one more watering for 15min. Three waterings during the 12hour light cycle. Many strains can handle every three hours.
Edit- just finished this journal. Nice grow.