:iagree: There is no set rule on how much corals grow because everyone is different. One SPS coral (Green Slimer) grows as fast as xenia.. about 1-2" a month, were as another one that looks very similar (Lime in the Sky) will take 8-9 months to grow the same amount.
Then like fsn77 stated.. so many variables like water/parameter stability. If your Alk/CA is bouncing all over the place corals will have a hard time growing, but if it all remains fairly constant, they will grow much faster.