The best way to tell if you need to shorten the time between waterings is watch your plants and if they show any signs of wilting at the very top of your plants then that is a clue that they have run out of water in the rockwool. You should water them before they get to that point.
To see how long your medium holds water just water your plants and then turn your
pump off and see how long it takes for your plants to start to wilt. Lets say they didn't show any signs of wilting for one hour after their last watering. Then that would tell you they should be watered every 45 minutes. That way they would get plenty of water and still have a semidry period that a lot of plants need for healthy growth.