I would have to agree with the combined posts of these two - fishieness and ThaNgBom.
Coralline Algae doesn't alone define LR. Your definition may be different, but mine would also include Aerobic and Anaerobic bacteria the true filtration and one most important advantage of LR followed by buffering. This doesn't require light as the Algae does. How long does it take? Good question! What I am typing next is IMO and not from any Marine Biology degree. Aerobic, the same time it would take to cycle a new tank, 3-8 weeks (too many factors to consider), then some time after that for Anaerobic. I wouldn't think it to be out of the question for base rock to become complete fully functional LR for at least 4-6 months with LR already in the system. Again, too many system factors to put a specific time table on, but I believe it would be months not weeks.