Like several others here, we feed twice a day.
I feed small portions, and I monitor them at every feeding to be sure that all of the food is consumed. If it is not, I cut back the next time I feed. Currently I alternate between flake, some frozen Angel Formula, and frozen brine that is enriched with spirulina. I have been looking for Nori sheets but can't find any, so this weekend I am picking up some Seaweed Selects. Just as a treat, I occasionally give a leaf of Romain Lettuce or hold a floret of broccoli in the water and let my angel nibble on it. My fish also graze constantly on the live rock in the tank. All of my fish are very fat, needless to say.
If they don't eat twice a day, I feel guilty. I have a juvenile Cortez Angel that acts like a dog...he rushes to the front of the tank every time he sees me or my husband and frantically swims back and forth. (All of my fish...the Cortez, a Flame Angel, and two Perculas...always group at the front of the tank and pretend to be starving.) He chews on my fingertips if I dip them in the water!
My tank is fairly new, so since I do feed twice a day, I am sure to test my water every 3rd or 4th day.