That was just an example mate... hence why I said 'for example'.
I totally agree, it can be very fast for microbes to propagate. That's why one can start with an almost bare tank, and after a short period of time (days), it could be filled with microbes. However don't forget that they can't always double their numbers easily though. Remember in biology class, that growth curve? Competition for food for example, is a major factor in limiting growth. Sometimes it won't be a doubling.
But why are we even talking about this? Sure, microbes can grow very quickly, but like you say, it does take longer to go from a few billion to trillions, for example. To be precise, going from say a billion to a trillion takes about 80 hours or somewhere thereabouts, as iirc the doubling time for our microbes of interest is 8 hours.
That is the reason why though, you do see many tanks that have gone to zero ammonia or whatever, but then when they add fish, see a large ammonia spike. That's because although the microbes are dealing with the ammonia, firstly there's not enough to take it down quickly, and secondly although they may be multiplying during that situation, they're not doing it fast enough. Which is why I advocate getting the microbes to be at high levels BEFORE adding fish, so that even in the worse case scenario, it's just one or two doublings (or whatever amount of increase it is) to be able to take on the nutrients.
Of course, it is entirely possible too for one to have plenty of microbes from the beginning, and it won't matter for example.
But see, that's also why I advocate testing for it. It's a simple test, and doesn't hurt to try. If there is enough microbes to deal with whatever amount of food we want to feed daily (plus some extra for good measure), then awesome. If not, then precautions may need to be taken. I guess if it's just slightly longer than a day, then that's fine.
Let's say for example someone set up a tank and added food or left it as is or whatever, and a week later parameters zero out. They have a 10 gallon tank and plan to add about 100 tiny pellets a day. They test 150 for good measure.
Situation A: The tank takes more than one day to reduce ammonia and stuff to zero, or close enough. Maybe it's three days.
Would it be good to know then? Since that would mean that the amount of microbes is not quite there, if it's gonna take that long. If they had added live stock such as fish right away and kept up their daily feeding, then by the second day there'd be all that ammonia and stuff from that day's feeding, plus whatever is left from the previous. And so on.
Situation B: The tank takes less than one day to reduce ammonia and stuff to zero, or close enough.
Well there we go, at least it's entirely sure that the aquarium can deal with a whole lot of food. And so when live stock such as fish is added and fed, there's no worries! Whether or not one wants to add all the planned fish, or multiples, or one a time is one's choice, but at least the microbes already all there to deal with it.
