In the Epilogue, Diamond is discussing possible reasons why China lost its status as a major power relative to other countries in the world. He does not discuss any exact time frame. He starts his discussion for the most part in the 1400s but does not give it a specific ending date.
Europeans never completely conquered China in the sense of making China a colony of some European country. China was always ruled (at least officially) by Chinese. However, one can argue that the British conquered China during the Opium Wars. These two wars happened in the middle 1800s. They resulted in China being subjugated in important ways. China was forced to accept unequal treaties with the West and to give Western countries (and eventually Japan) major concessions. To this extent, they were conquered even though China retained its own government.