Providing you don't have fry, once a day is typically enough. Do you have any fish that are more on the nocturnal side? If so, feed right before you go to bed. If all your fish are up during the day, just a "small pinch" of food in the morning plus whatever your bottom feeders like and you're good to go. You don't want to turn the tank into a giant cloud of food- that's too much and just makes your nitrate rise faster, plus more to vac out of the bottom every week when you clean. The fish should be going to the food, not the food going to the fish.