Pilots FDP in Hongkong or around the world

Dear pilots,

Im currently working as ground employee,
I would like to know how does FDP (flight duty period) operated in hong kong, especially for cathay pacific pilots.
From my research, its time between duty report time and engine shutdown.
I’d like to know how it is practically calculated, because sometimes HQ says ground work has to be done faster than usual due to FDP restriction.
If pilots should know how many hours of FDP left for their duty, then what is the rule? Per week? Or per month? How many hours?
And what happens if taxi hours become longer so it gets very critical?