I think the key moment that central government is waiting is the point where very large number of Hong Kong people walk up the street for counter protest (against the rioters). So far, that has not happened, only two moderately "large".According to my girlfriend, several local gangs can no longer tolerate the riots and have beaten up quite a few rioters who immediately ran away and called the police. The police did not arrive on time and now the rioters are accusing the police of failing to protect them. You can't make this up. I used to believe what Josh Luo said, that they were willing to die for their cause simply because they seemed crazy and brain-washed enough; but now I don't believe it. Not only do they run away when the police defend themselves with guns, they run from gangs with sticks, gangs that they far outnumber. These are the actions of cowards who are only willing to fight when they know the other side is obligated to not harm them. They will not die for their cause; they have shown on every occasion that their self-preservation is very much intact.
I imagine that the people of Hong Kong are getting more and more fed up but I am seeing this change personally as at the start of the conflict, my girlfriend called them protesters, not rioters, for which I corrected her profusely. Midway, she said she wished the PLA would come sort them out and save Hong Kong with non-lethal force, arresting the hidden ring leaders because she says most of the people are just confused and stupid. Now, she's cheering for the gangs. I said, "I would love to see them defeated forcefully but China's enemies would love to see Chinese people kill Chinese people," to which she responded with a reptilian coldness, "Bad people need to die." LOLOL We've essentially reversed roles
The point is "one have to show the will to save oneself before asking for help". Hong Kong people must show clearly that they want it stop, and order being restored. They must show that they support the Hong Kong government and police force. Only then and only when Hong Kong police force is stretched thin would central government intervene. That is a mandate (from Hong Kong people to Beijing) necessary under the "One country two systems" arrangement. If there is no "two systems", the whole thing would not has happened.
Hong Kong people can NOT hide behind Beijing, demanding Beijing to be the "bad" guy while they can pretend to be the considering "nice" guy.
About the "gangs" beating the rioters, if my life is being ruined by the rioters, if I can not feed my family because some idiots want "independence", if the police can not protect me, I will do it myself. Just like the 1992 Los Angeles riots.
A word to the Hong Kong government, if it can neither suppress the rioters nor protect the people who is not part of the protest, it (HK government) will loose support from all sides. Historically many government fall due to their incapability. Another moment when Beijing has to intervene.