Reading in FB forums, it seems that PCR normally takes between 1 and 10 minutes. It depends on the gate where your flight land. I recently landed near immigration but had to walk back to the PCR test station.
If you are unlucky, there could be heavily loaded planes landing just before you, creating waiting time at the PCR stations, but the line moves quickly. That will happen when pax return from the current holiday. To be safe I would budget one hour.

I am sure that you are aware of the risk of a positive PCR (results with a couple of hours) that might trap you in HK or give you trouble if you return.