splifman is too right about rockwool holding lots of water. I use 8" plastic pots stuffed with loose rockwool (aka 'floc' or 'granulated' rockwool) in a flood system. I only need to flood once a day for 5 minutes. Small plants (under 2 weeks in the flowering area) need less water. Depending on conditions, I may flood them 1x/day for 1 min or 1x/2 days for 2 mins.

I have the benefit of having 4 flood trays, each with its own tank, pump and timer. Clones go in every 2 weeks and plants ready to harvest come out. This way, each tray contains plants at 0-2 weeks, 2-4 weeks, 4-6 weeks and 6-8 weeks in flowering. The timers can be adjusted for the differing water consumption rates. Plants at 4-6 weeks are thirstiest. This arrangement makes it easier to flush plants with plain water in the last week as well, without altering the diets of all the other plants.