Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Some soils take as long as a month to settle down.
There are many microorganisms in soils. Some adapt to living under water very well, and multiply. Some die, releasing ammonia and other things. If you are starting with garden soil, make sure you can give it a month for the microorganisms to make this adjustment.
Packaged soils also go through a similar process, but often they are sterilized so they start with no microorganisms. The bacteria, fungi and so on start growing once the product is under water, and this can take a few days for the fastest species, but more like a month for the slower species. Since they are all working together and the system is changing as the population changes, I would still give it a month to settle in. You can do the fishless cycle during this period, and have the new tank ready for the fish.
If your goal is to change over an established tank, then I would set up the soil in a shallow container of water and circulate the water, and test it to see if it will be releasing ammonia. If so, keep it in that container, water circulating, to get it ready for the aquarium.