1784 at the latest, but the Chinese government and relevant merchants would have probably been aware of the American Revolution as it was occurring. The Colonies were a major market for Chinese goods (notably tea) through the British East India Company.

As for how they took the Revolution, they just weren't very interested. Qing China ruled roughly a quarter of the world's population and a disproportionate amount of its wealth. Foreign affairs would have been a relatively minor matter until the First Opium War (1844) and the US was a minor player in the region until the Philippine-American war (1899).

Fateful Ties: A History of America's Preoccupation with China by Gordon H. Chang