3 flakes sounds like very little food for 17 tetras. I only have 5 and they eat more then that in less then 2 minutes. The reason you get a lot of different answers is because different fish have different needs. I've found tetras to be the more go getters of the fish in that when they see food they rush to the top to eat their fill and shove all others out of the way. So they have usually consumed enough food in about 2 minutes. Now with 17 fish, the bigger fish will probably be pushing their way through and the smaller ones might be left behind a bit. So adding food for a few more minutes (like the 4 ATM suggested) might be in order so that everyone gets what they want. The best way is to watch them. They usually will have a few that will actually feel full and back off a bit. Once you start to see this in enough of the fish, you can stop feeding them because the others are just eating because its there. Find the right mix for you, and also if you haven't already, look into another food for every now and then. I use
freeze dried blood worms as a treat once or twice a week. They love them and it helps vary their diet and give them a varied source of vitamins.