Honestly, the minute the bleach water hits it, it should kill anything. But I would let it sit for a few minutes, give it a good scrub, then scrub it again in some dechlorinated water (not RO water, dechlorinated water so the dechlorinator will deactivate any bleach still on the rock). Scrub until you get the algae off.
The longer you let it sit, the more reaction the bleach will have. It may discolor the rock the longer you leave it in there.
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