Its a judgement call. Long enough for the corals to ingest the food. I turn my return pump off, and hit the food timer on my tunze. That gives them 10-14 mins to eat then the pump comes on and blows what they missed around real good. Then I hit it again and let them eat what they caught, turn the return back on and let it go at that. I only feed rarely though. But I have MH so they get most of what they need from fish waste and light.