Flood/drain- rockwool/hydroton, how often to flood

I started my clones in 1 inch rockwool cubes, now there in flood/drain buckets, rockwool cubes sit 1/4 inch into the flood line, how often should i flood?, im worried about rootrot for keeping the rockwool saturated, they are in veg, 1st flood 2hrs after lights on 2nd flood 5hrs later , last one 5hrs later , 3 floods a day, plants are 8 inches tall.. 15 min floods.. anyhelp appricieated

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