When we first started out- every week, then in late veg (after 8 weeks) we let it go two weeks but topped off with half-strength nutrient.
Ideally you would want to put as much as the plants would drink in 7 days; some even recommend 4 days for maximum growth. Say the plants drink a gallon a day- you mix up 10-12 gallons of water (because the pump needs to be submerged in the res and you need at least 10 gallons in there for the pump to not run dry) and then just add a gallon of fresh water a day, up until you add as many gallons as you originally mixed as nutrient solution.
We might let the plants go on this res for a few extra days so they are definitely in their second week of flower when we hit them with the Big Bud and some Colossal Bud Blast.
They have enough to go about 10 days (35 gallons of nutrient solution)
I've been advised to flood 5X a day, every 4 hours with the lights on and every 6 hours with the lights off. This is to get as much oxygen and fresh nutrient to the roots as possible. The fresher the nutrient and the more oxygen the better the uptake.
You might want to skip the grodan and just use hydroton, the more flood cycles and the more the nutrient is refreshed the better.