It is a hard to say exactly when, as it will depend on several possible triggers. The CCP have adopted a belief that nothing can possibly go wrong with any of the "projects" it has undertaken. As these projects inevitably fail, the party will be seen to be weak, and thus lose legitimacy. This will also lead to the collapse of the party.

One major project that will cause them grief is their claim on the South China Sea. The government's legitimacy rests largely on their success in this claim, which the rest of the world can see is doomed to failure.

China cannot see any possibility that the UNCLOS suit Philippines have brought against them can go ahead, or that it will be successful. If it is successful, then China will have to fight a war to enforce its claim. If it doesn't enforce its claim, the people will see it as weak, and it loses legitimacy. If it does fight a war, it believes that it will be a short and sharp war, as they know they cannot succeed in a long and protracted war. It will inevitably lose this war, and the resulting sanctions will destroy their economy. Again, the government loses legitimacy and power.

If the Philippines case is not successful, the US will not allow China to have control of such important trade routes. Again, China must either back down or fight a war it cannot win.

The alternative they are trying to create, in case of losing the South China Sea, the so-called "New Silk Road" through Pakistan is also doomed to failure. The country is politically unstable, and hostile forces will disrupt effective operation of this as a trade route. A change of government could also see China expelled from Pakistan, just as a change of government in Sri Lanka saw the new government cancel plans for China's "Maritime Silk Road" project in that country. Again, the government loses legitimacy.

There are many such "projects' that China has gambled its legitimacy on, all of which China cannot conceive will fail. The failure of one or more will prove a loser for the CCP, and that is the time that their power will finally come to an end.