For an 8.5 hour layover in Hong Kong, what amount of time would one actually have to do something in the city?

How long would it take to get from the plane to the city (including immigration, customs, transport)?

How far ahead of time would one need to depart from the city to the airport to get to the gate safely in time to make the flight (including transport, security, etc. -- what exit emigration and customs procedures are there?)?