This is the hardest question to answer because every situation is different. One & Only will dramatically reduce the time it takes to establish a biofilter in your newly set-up aquarium. However, even One & Only has limits.
For instance, if you set up a saltwater tank and use uncured live rock, One & Only may not be able to initially control the ammonia concentration in the water because there will be too much decay of organic matter from the uncured live rock.
Or if you let every friend of your kids feed the new fish tank such that you are grossly overfeeding the tank, One & Only will not be able to maintain low levels of ammonia and nitrite.
Also, some fish species are very sensitive to any amount of ammonia in the water so these fish should be added to the tank in the first 20 or so days after set-up.
In general, you can add one giant danio-sized fish per gallon when initially dosing with One & Only. Feed normally and monitor ammonia and nitrite levels in the water. You might see a little ammonia or nitrite for a day or so but this will quickly disappear. You can then add more fish to the aquarium.