depends on a few different things. lights,type of RW. but ive always had good luck with 2 -3 ,15 min intervals. on botanicare trays with only 2 holes not 4 also running with 2 600 watt hydrofarm air cooled reflectors........try 3 settings of 30 min and if its to much water, work your way down.i would rather have over waterd plants then dead dried out plants