I have seen posts where people complain that their rhom was up to 9 inches and didn't have red eyes yet. I guess it's up to the fish. Like they said above most will start at 5 or so, but others may take longer.
I would suggest posting a photo of your fish to be sure it is S. rhombeus. The eye color is very stable on this species and one could say distinctive at most ages, though some exhibit more of an orange than actual red color on the orbit. Water conditions are primary source of this coloration.