If i take taxi from Dongguan to HK, how much and how long will it take?

The distance is about 80.7 mi (130 kilometers) between. To catch your 21:10 flight, your time would be very limited if leaving aroun d 17:30. So the best choice would be to take ferry. You can take a taxi to DG Humen Ferry Terminal, and take ferry from the terminal to reach HKIA directly. Five ferries are available at 08:15, 10:15, 12:00, 14:45, and 16:00. It takes about 70 minutes to reach. So I suggest you change your departure timing a little earlier.

Taxi is hard to rent, but you can book a private limo to reach. It would cost around CNY1,200-1,400. It would take about 1.5 - 2 hours.