how much time I need for ferry terminal

You need to firstly make sure that your flight at HKIA is eligible for the ferry route.
It's enough to catch the 7pm ferry as the ferry departure should be 2.5h in prior to the flight departure. You can buy the ticket at the port and should check in the flight and get the ticket 60min before the ferry departure. You will need your flight no. to buy the ferry ticket.