Look at it as a food chain thing. Those fish attractors grow algae. The plankton feeds on the algae. The bait fish feed on the plankton. Then you know what happens from there.
The timeline is different with each brush pile/fish attractor. If its organic (a real brush pile) the algae will grow more quickly and you will likely see fish holding on them sooner. But once the plastic ones get some algae going their just as good. but to answer your question......how ever long it takes.
If it ain't yours don't mess with it!
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