Rockwool holds water longer than
grow rocks so I wouldn't water as much. Maybe every couple of hours or so. Only let the
pump run long enough to wet the rockwool and then let the solution drain back to the tank unless you have to use a timer that runs longer then you don't have much choice but to let the pump run the minimum on time of the pump. A Dayton 1or 2 hour Cycle Timer from Grangers is the best but cost about $80.00. For serious growers you will have to buy one of these no matter what they cost.
Ron