The Forbidden City was home to 24 emperors - 14 from the Ming dynasty, and 10 from the Qing dynasty. When the last Emperor of China, Puyi, abdicated in 1912, the Forbidden City ceased being the political center of China.
While relatively large, you can spend a lot or a little time visiting depending on your interest. I think a couple hours is good enough to get a good feel for the place, but 3-4 hours is not unrealistic either. If you're like me, taking a million photos, you might need a little more time.
Inside the walls of the Forbidden City, a UNESCO World Heritage Site.