Hi Sally,
It takes about an hour by car to the airport from Sai Kung. By public transport, you can get a bus to Choi Hung and then MTR to the Airport Express Station - could be an hour and a half or even longer for this.
There are various airport buses as well... which one of the following you take would depend upon where you live in Sai Kung.
For South of SK Town - you'd get a bus to Po Lam station; north of SK Town - Ma On Shan or Shatin are your best bets.
A41Sha Tin (Yu Chui Court)
A41PMa On Shan (Yiu On)
E42Sha Tin (Pok Hong)
E22BTsueng Kwan O (Po Lam)
The time it takes is heavily dependent upon time of day, of course, but - I'd guess at 1.5 hours until very early morning or late at night.
Cheers!
Judy
http://www.exploresaikung.com