When they start making a mess of them. I switched my dwarves about the 2" mark because they were destroying more flakes than they were eating at that point.
I don't know why people even start their fry out with flakes... My cichlids have never seen a flake in their life. The pellets soften up as they sit in the water which let's the fry feed at will. Its also easier to remove the rest of the pellet/pellets that did not get ate vs flake food.