I am not sure where that 12 hour idea started, but I fear it might actually have been with me some years ago, I hope I am wrong.
I recall making a post years ago where I suggested that one might also add only a 50% dose of ammonia and then test in just 12 hours. Because a tank neither creates nor processes ammonia at a constant rate over a 24 hour period, this can be a bit less accurate a method. It is important that most of the 12 hours be during the most active period for the fish. In most tanks, fish are more active during daylight hours, so therefore should create more ammonia then. However, itt is a good idea to know when one's tank is likely to produce the most ammonia and test during a time when it occurs in their tank if using the 12 hour method. This means a tank with a lot of nocturnal fish would need to be tested over night as opposed to during the day since the fish will be making most ammonia then.
Using the 12 hours test was just a way one might speed up the process of insuring the tank could likely clear double the ammonia in 24 hours.
It helps here also to understand the difference between how an established tank produces ammonia vs how we add it during a fishless cycle. In the first case, the ammonia is continually produced 24 hours a day. However, the amount of ammonia produced in any given hour is only a fraction of the total ammonia produced over a full day. When we do a fishless cycle we are usually adding at least a full days worth of ammonia production in one single go. To this extent fishless cycling and cycled tanks are totally different.
Moreover, the total amount of ammonia most tanks can produce over 24 hours will be less than the full dose we use to cycle. The reason for this is simple. It is better to start with more bacteria than might be needed for the fish you are about to add than to have it be too little. In addition, most African rift lake cichlid tanks rely on overstocking as a way to blunt aggression. They are also usually higher in pH than average which makes ammonia that much more toxic. As a result, such tanks need more bacteria than others. Additionally, it is not uncommon for new fish keepers to make the mistake of overstocking a tank, sometimes on the advice of the place selling them their fish. So any fishless cycling method needs to insure such tanks will be safe.
I usually do not suggest to people doing their first cycle or two that they use the 12 hour test. Stick with the 24 hours test and a full dose and be sure.
One note here: There is no reason one cannot add a full ammonia dose and test in 12 hours. While it is not expected, some tanks will clear all the ammonia in that time and if so you can then test for nitrite immediately. If that is also at 0, you are cycled and don't need to test in another 12 hours. But if you are not 0/0 in 12 hours, you do need to test again and it is that 2nd test that is the critical one.
Your tank is cycled. Do a big water change and get some fish into it