If you use a filter or filter media from a well established tank, as soon as it is set up and to proper temperature it should be safe. Keep in mind waiting too long is actually dangerous when using media this way, because the nitrifying bacteria will starve without food, and that you should start with only a few at first to give the bacteria time to colonize the new tank.
2-3 days if you condition the water/use rocks and plants from an established aquarium.
I wait a week
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