Pebbles...make sure you rinse the 100 times!!!! OK I exaggerated but rinse the dust many times or it will mess with your pH and cause problems.
Using flood dripper
15 on 45 off lights on.one 15min on in middle of dark period.
When pots fully rooted 15 on 15 off lights on. Keep one 15mins on in middle of dark period
Coco
Hand water as per soil for 1 to 2 week or until roots hit the btm of pots.
When pots rooted And using drip dripper
15 mins a day at beginning of lights on week 1 veg
15mins on every 6 hrs lights on week 2
Up to 4 x 15 mins lights on when pots full of roots
Maybe it could be useful: you can set the irrigation always on (obv. in a recirculating setup) unless the dissolved oxygen level is too low.
Roots don't care about too much water. Water rich in oxygen does NOT carry too much anaerobia bacteria and provides the plants enough oxygen to have an happy life.
Be sure to feed enough oxygen to roots...the way it's up to you. There are many effective ways to do it.