Flood and Drain system when using Rockwool it does hold alot of water.
You should only need to flood once a day for 5 minutes. Small plants (under 2 weeks in the flowering area) need less water.
Depending on the conditions, You may flood them once daily for 1 min or once every 2 days for 2 mins.
Different shapes and types of rockwool will require different watering. The trick is to keep the medium damp, never saturated or wet. Flooding rockwool several times a day sounds unneccessary to me for any shape other than very small cubes (4") used in NFT or aeroponic systems, which might be expected to support a full sized plant. Something like that would require frequent watering, but not flooding.
Slabs are usually smaller in volume of rockwool per plant than an 8" pot full of loose rockwool (floc), so my figures from here may not work for you.
An 8" x 8" pot of floc will soak up about 2 litres of water. That's plenty for a roughly 30" tall plant in mid flower or active veg (ie mothers) for 24 hours, plus about 12-24 hours in reserve. Nice if a pump fails and you don't check daily.
The trick is to match how much water you give to how much the plant uses. It's a bell curve. There's too little, just right and too friggen much. Most newbs overwater plants to death. Yellow leaves? Slow growth? Back off the watering.
Watering once a day is fine if your medium will store enough water for the plant for that time period. Take a block of rockwool or a pot of floc, weigh it, note the dry weight (in grams). Add water until it drains freely and allow exces to drip away for about 10 mins. Weigh again. Subtract the dry weight and you know how the weight of the water. Since 1g of water = 1ml = 1cc, you know how many ml/cc of water is in your medium.
Put the wet medium under your lights. Check the weight a day later.
If the water weight has reduced by 50-66% in 24 hours, it would be OK to water again.
If the weight remains above ~50-60% of the original wet weight, roots may drown, develop fungus probs, etc. Either give less volume if handwatering or flood for a shorter period with your timer. Dial it down until your pots are fairly light by the end of 24 hours (25% of saturated wet weight).
Ideally, the plant will suck up 50% of the total water in the medium in 24 hours. This should keep root probs at a minimum and give max growth.
Contrary to newb belief, watering more than a plant needs doesn't force a plant to grow any faster or better. Usually kills 'em. We're not talking turbochargers and fuel injection here. With plants, there's right and there's dead.
