In that case, the answer is that it is typically determined by the validity of the police clearance and medical, and the first to expire.
So police check was done in November, my medical in January, and my visa granted in May with a must make first entry by date of November, exactly 12 months from the date of my police check. That meant I had around 6 months to enter Australia. Some people get more of the year, others get a couple months or few weeks.
This is another reason why it is a good idea to wait until requested on those items rather than submitting them early... Not just the risk of having to do them again if they expire, but also you don't want to limit your first entry by date if you can't just pick up and leave once given the grant.