Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Depends on how big a population of bacteria you want. They start colonizing it right away, but are slower growing than many microorganisms.
I would give it at least a week just to get a starter colony going.
A month would give it a much better population.
If you need bacteria right away the better way is to swap some filter media: Take media from a well established tank and put it in the filter of the new tank. Put the new media in the old tank.
You can swap them back after a while, but as above, I would leave it at least a month.
Then the new tank will also have some bacteria building up on the surfaces inside the tank, too.