Base rock added to an existing tank will become LR
in a matter of days. All that means is that nitrifying bacteria will start to grow there. The same is true of almost any object that is placed in the tank.
If you are looking for Coraline algae growth, that depends on lighting (it's a low light algae), water parameters and the presence of Coraline algae already in the tank. It could weeks, or months.
Do you know the full history of the rock. Has it ever been in a tank that was medicated with copper or anything else?