Originally Posted by Globalist

I have a different perspective here.
In town check-in is regularly used in the morning for evening flights and stay in town for work or pleasure. In this way there is no presumption you immediately take the train.

So to the question, is there a minimum time you will be allowed to enter security/immigration? I don't know.
LHR has 35 minutes conformance, I have not heard the same for HKG. Perhaps others can address your specific question.
I personally would like to enter 1 hour before departure it can easily take 20 minutes on a normal day to pass security and immigration, take 10 minutes to the gate at which time boarding most likely has started. (That said, 3 hours is great to enjoy the nice lounges).

As for the protests, I understand that there is now a ticket check to enter the check-in area to ensure only passengers enter the airport. Looking at this and the ever changing situation in town expect queues at places where normally are none, safety is paramount and you want to take in extra time to reach your destination. Build in buffers.

Globalist