Please discuss this with your visa advisor, as there is no fixed time when you should progress these. If you have had a past criminal conviction you may want to obtain a police certificate at an early stage to establish what information is on your file – police certificates tend to be relatively cheap.
However, medical examinations typically have a 12 month validity (see the above question regarding the date of required initial entry to Australia) – and aren’t cheap: attending these too early can mean you have to do them again.