To what extent does black tea ferment?

Black tea usually ferments for 4 to 6 hours. But the specific fermentation time depends on the age and tenderness of tea, the hot and cold weather, the withering and drying degree, the wetting degree and the kneading degree. Generally, young leaves, fully twisted raw materials and leaves with high fermentation temperature have fast fermentation speed and relatively short time. On the contrary, it will take longer. I have encountered 12 hours without fermentation. As long as it is not sour or stuffy during fermentation. Tea makers should keep track of the fermentation progress at any time. Fermentation time is only a reference index. Whether fermentation should be stopped depends on the degree of fermentation. Moderate fermentation is generally based on the fact that the leaf color basically turns red and yellow, the green gas disappears, and the flowers and fruits are fragrant. If it is mass production, the final evaluation method is to look at the color, taste and leaf bottom of the soup and check the degree of fermentation. The book says that temperature and humidity, a few hours, are routine. Top tea makers only say "watch tea and make tea". The road inside is very mysterious!