Well, that just sounds like a normal observation someone would get along the way while doing a fishless cycle. It sounds like you at least have the chance that its proceeding very quickly and nicely compared to most. Did you get hold of some mature media?
If I'm reading your sentence correctly, it sounds like you observed just ammonia having dropped to zero in 12 hours while the nitrite reading is still at the max the test can give you at 12 hours? That's pretty common, it that's what you observed. The N-Bac development is much, much slower than the A-Bac development. Sometimes the A-Bac development will even backtrack some in the coming period while you wait for nitrites to drop.
If you meant that -both- ammonia and nitrite suddenly dropped to zero in twelve hours then of course that's even better, and you would then just keep adding to 5ppm ammonia at the 24 hour time block and taking readings at the 12 and 24 hour marks. If you get several 12 hour drops to zero then you could go ahead and start your "qualifying" week to see if it can keep it up or if it will suffer a setback.
~~waterdrop~~