How long is a transit visa issued at Chinese airports now?

They don't issue any. In China, a transit visa cannot be obtained at the airport; it is not provided. For transit through Chinese airports and ports, citizens of Russia and other countries can obtain a visa-free transit. Visa-free transit is a fundamentally different type of permit document. It is free and currently allows you to stay in China without a visa for 240 hours. More details are described here - https://nashaplaneta.net/besopasnost/novye-pravila-tranzita-cherez-kitaj-10-dnej-bez-vizy

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