I think do study-on questions, checkpoints and anything of the like during the year. In reiterating what the person above me stated, it's good to start having a crack at some exam questions after you get a grip on the content. Study-on and checkpoints tend to categorise questions into topics so you don't have to search the site/book for hours trying to find suitable questions. I found they were good for SAC revision and a 'gentle' introduction to exam style questions.
As for practice exams ... I think they're best later in the year. Essentially in an exam you are showcasing a combination of knowledge and skills learned throughout the year which is probably best practising after you get a firm hand on all of the content and key skills to maximise score potential and learning potential from each exam. Some subjects like biology and chemistry, for example, used to have mid-year exams for unit 3. If you're keen, you could try those exams in the middle of the year and are still on the VCAA website I believe. Some external companies also have specific unit exams, like TSSM, but can be a bit pricey.