The bacteria colony can double in size in 24 hours, so I have always used that as a guideline. What that means is the bioload that you can safely add to the tank is a function of the current bioload of the tank. The larger the colony, the faster it can grow. If you have 20 neons in a tank, then theoretically you can add 20 more and have things balanced out after just a day or two. In practice I have found that to be more or less accurate - I've combined two tanks of similar bioloads (adding some 25 fish), dosed daily with prime for 3 days, no water changes and no problems whatsoever.
Of course it's important to have the filtration capacity to allow the bacteria to grow.
Of course it's important to have the filtration capacity to allow the bacteria to grow.