Just a few days is all it takes. They move a surprising amount of water. I have had one in for 2 days and then started a new tank with it and never had any WC issues.
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From my experiences I would run it a good week, maybe even two if you plan on moving it to a tank with a decent stock. BB does take some time to actually stick to the material so its always a good idea to give it some extra time. I run filters on existing tanks for a week and they can handle light/semi moderate stocking.
I've heard people say bacterial colonies need as much as 3 weeks to become fully active in new media. I've been kind of going off of that for some time, however I have no evidence to validate that rule... but its a good "better safe than sorry" kind of deal
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