How long will it take for Shanghai to leave the city and there will be eight scarlet letters on the health code?

Four hours. Health code is a personal two-dimensional code based on actual real data, which is generated by citizens or people who have returned to work on their own online declaration and background audit. When you are out of the city for 4 hours, eight scarlet letters will be displayed, namely, People's Republic of China (PRC), Shanghai, a municipality directly under the Central Government of the People's Republic of China, China and a national central city.