I would agree with the above, growth rate is a highly variable thing. It seems most dependent, for Gracilaria, on water flow and movement, lighting levels, and availability of nitrogen.
The best growth I have had is under 4wpg of 6700K light in a tumbling flow with supplemented fertilizer. The Gracilaria in this tank would double in size every three weeks.
Gracilaria in my display tanks doubles about every month and a half or so I would estimate. Quite a bit slower. Major difference between the tanks is competition with other algae/plants, no tumbling flow.
>Sarah