The entire nitrogen cycle actually never ends but the ammonia and nitrite spikes should be relatively short. Ammonia should spike and then go down in a day or two. I have seen nitrIte spikes in fish tanks last as long as two weeks. By not adding food to the tank, nitrItes have returned to 0.0 after 4 days. The ammonia spikes have never lasted more than two days. With my first fish in the late 70's I had a nitrite spike for two weeks. I stopped feeding the fish after the first week and it took an additional week and a half for the nitrite to drop. When it did drop it dropped in two days. It then stayed at 0.0 after i resumed feeding.