Help me.Confusing with the time.N21 in Hong Kong

I 'm staying at Chungking Mansion,TST Nathan Rd.My flight leave at 6.45am.(leaving on 15 /2 2014).i have to be HKIA by 2 to 3 hr early. I understand the N21 bus stop operate here.Can you tell me what time will the N21 bus be at that area.Do the arrive at 4am to 5am. so I can catch the bus to airport on time.Reply ASAP Thanks