How many floods is too much?

Hi-1st time with an ebb n flow and a SOG-

I've got 25 plants in 2 gallon bags. I've got the bottom of tub lined with some clay pebbles (is that necessary, not sure reason why-was told to, not hurting anything?), and filled with perlite/coco mix. How often do I flood and for how long? Right now its twice a day with 1 click and 2 clicks on a mechanical timer. Is the right and should it stay the same through out the grow?

I'm really loving the low maintenance of it, thinking about adding a float to res so I only have to open tent 1 a week for fluid change.

Nobody floods coco?

Depends on the size of the plants. Likely once or twice a day in coco is all you should need.