> How long would it be until their entire fleet is sunk? 2 days? A week?

That's not something you or anyone else could predict.

> How long after that should the Three Gorges Dam be allowed to exist?

This would immediately cause a death toll in the millions and a nuclear response would follow.

> That's not something you or anyone else could predict.

You're right: nobody can possibly predict how a nation with no blue water navy, and zero relevant (naval or otherwise) combat experience (ever) might fare against the most expensive, trained, veteran naval combat force in the world - backed by the world's two largest and most expensive, trained, veteran air forces (USAF, USN).

> This would immediately cause a death toll in the millions and a nuclear response would follow.

This presumes that the United States would ever allow a blockade to happen to begin with. The US Navy goes where it wants.