Well, the very first time you dose the tank (to 4-5ppm) it can take a long time before the ammonia finally reduces to zero ppm. Sometimes it will go down a little the first couple of days but then just sit there for a week without doing anything. In a few terrible cases, it might just sit there for 2 or even 3 weeks, but that is rare. Usually after 3 days to a week and a half or so it will start going down and you'll get your first zero ppm reading (all this without you having added another drop of ammonia, just that first dose.)
But then when it drops to zero that first time you may find that that the next time it goes faster and then faster still, until quite quickly it is dropping to zero ppm within 24 hours.
Once it starts dropping like this you want to have decided on your "add-hour," which is the regular hour (out of the 24 hour day) when you will add ammonia (if indeed it is needed, as indicated by it going to zero in the preceeding 24 hours.) You don't add ammonia until that hour, even if you measured it at zero ppm earlier in the day. Letting it fall to zero ppm can help stimulate the bacteria to do well.
At some point, usually after ammonia has begun to drop and is showing that you've got some A-Bacs in there using it up, you will get your first measure of nitrite(NO2) showing up. In fairly short order you will usually get a -lot- of nitrite showing up and when it hits the highest number your kit can measure (such as 5ppm or so) we refer to that as "the nitrite spike."
The beginning of the nitrite spike marks the 2nd of the 3 phases of fishless cycling. During the second phase of fishless you can reduce your ammonia dosing to 2-3ppm rather than 4-5ppm. This reduces the amount of nitrogen you're pumping in to the overall system and can help delay the need for big water changes at some later point.
This "nitrite spike" phase can take even longer than the first phase did and it doesn't end until the nitrite finally comes down and reaches zero within 24 hours of when ammonia was dosed (yes, the ammonia just continues to drop pretty much each day and you re-dose at your add-hour each day, but you keep measuring and keeping a good log of all measurements.
Once the 2nd phase has ended and nitrite(NO2) is dropping to zero within 24 hours you are in to the last phase where your goal is to get it dropping to zero within 12 hours. This phase too can have its sticking points and sometimes be frustratingly slow but eventually it -will- happen - the ammonia and nitrite will both be dropping to zero ppm within 12 hours after dosing ammonia. In the 3rd phase it is important to ease your ammonia dosing back up to 5ppm so that your final bacterial colonies will be as robust as possible.
As the fishless cycle progresses, the final product of nitrate(NO3) will begin building up more and more. In water, the nitrate ion will be in the form of nitric acid for a small (about 7%) amount and that acid will need to be neutralized by some of your water characteristics or the pH will drop and the bacteria won't grow. Because of this the members will sometimes recommend that you do a complete water change late in the game (re-dosing ammonia after that of course.)
Once you hit your double-zero 12-hour drop you can start your "qualifying week" and hopefully just watch your now functioning biofilter repeat accomplishment each day. A final large water change gets the nitrate out and fish (up to a full stocking although almost nobody does that) can go in the tank. That chance also starts the clock on your new maintenance habits of the weekly water change and monthly filter clean.
Sorry that maybe did not sound simple, but you can look back and read it later when you need to.
~~waterdrop~~
