How can i get to shantou city if im in hong kong airport? How many hours?

When are you arriving at the airport? Anyway, there is one direct cross-border coach to the city every morning at 07:00. It costs 6h and the bus fare is 280HKD.

If you miss the bus, I suggest you take a direct bus to Shenzhen Luohu. Then walk to Luohu Bus Station nearby to catch a direct bus to ST, taking around 5h.

Also you can consider flying from HKIA to Chaoshan Airport and then take a shuttle bus to the city.