Depends on a few things:
1. Whether your medical has any required follow ups
2. When the next available batch is
3. Whether you are ab initio or advanced entry
Could be a week to over a month, just depends on the above said factors.
On another note, on the Cathay careers website, they have now changed the wording of the ab initio requirement to "HKID Card Required". It is no longer "HKID holders will be given preference". Guess that settles that then.