Approximately how frequent are the bus services from HKIA to Guangzhou.

Generally speaking, the interval is around 10 to 20 minutes.
The bus fare is around 200HK$ to 250HK$ and it takes about three hours and thirty mintues to four hours.
If you want to book the ticket, you can book on the official on HKIA. But in my opinion, it is not necessary to book in advance.