I recommend a varied diet. A mixture of meaty foods( fresh fish and shrimp from the supermarket or frozen from the LFS), flake, nori...
As far as how much, it depends on the amount and size of the fish. When I add some food to the tank, it takes the fish 20-30 seconds to eat it, except for the nori of course. That's attached to a clip on the glass and they tug on that for about a minute or so. I also give nori about every other day to my home system.
I also do large, weekly, water changes plus skim and have a refugium.
The tank I spoke of that I fed twice a week also had all of this going for it as well, but I could only get there 2 times a week so nori was added then too.