If you have a BN(O) passport, you can apply at any time you want. You do not need a Certificate of Sponsorship (CoS) from the HKSAR government.
On the other hand, if you only have a HKSAR passport, you will need to get a CoS from the HKSAR government. The current round of applications started on 20 February and it will be open until 5 March 2018. Once you got your CoS, you will need to get the UK visa before 9 July 2018.
http://www.whs.gov.hk/en/partners.php?location=uk