How often to water our hydroponics setup?

[JB]

I have a system that I designed that is a cross between an ebb & flow and a NFT system.

The plants are grown in 4" rockwool blocks, and there is a 1/8" of nutrient film that runs along the bottom of the blocks every 15 minutes, for fifteen minutes.

My question is:
Am I watering the blocks/plants too often?

hey seem to be growing healthy but they are a shade lighter green that plants that I am used to growing in soil. Although they are a lighter green, they do not appear to be pale.