How much time to spend at the Forbidden City: 4-7 hours
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I think if you have never been to the Forbidden City before, you need to plan 4-7 hours to visit it. It's best to plan an entire day for the Forbidden City and its immediate surrounding areas (including the Tian'anmen Square, Jingshan Park, and Beihai Park)
Approximate time breakdown:
15-20 minutes to walk from the closest subway station (Tian'anmen East or Tian'anmen West) to the front gate of the Forbidden City
15-30 minutes to buy tickets
5-30 minutes to get through security
30-45 minutes just to walk from one end of the palace to the other along the central axis, which is only the biggest halls
Another 1-3 hours to explore all the alleys and the private quarters of the emperor and other royal family members. There are also museums inside the Forbidden City that has a collection of everyday items used by the court
If you are in a rush, you can probably compress your schedule down to 3 hours, but this is very rushed and you will only have enough time to quickly walk through the main axis.