Guess I'll join the band wagon also. Rockwool should be soaked until the water you start with (5.3-5.8) is in the same range when you finish soaking. In other words PH water to specified range, put in RW to soak. After 4-6 hours check PH. It most likely drifted out of range so dump and refill PH'd water and soak, then chaeck PH. Rinse and repeat as many times as needed to get PH to stabilize in this soaking process. It all up to you. You can fix the PH drift issue while soaking or you can do it while your growing and hopefully not to far out of range for plants. Btw while using RW be prepared to do some flushing to remove salt build up on the cubes.
Have a nice day!