In year 11, when i did one 3/4 subject, I started doing practice exams in the second week of the September holidays (I did one in the first week too but I wasn't at home that week so I wasn't really concentrating on it properly).
I'd say given you're only doing one 3/4 this year, the September holidays is plenty early enough. I wouldn't be doing any full exams now, however you could start doing some U3 exams (pre-2013) as you get to the end of the U3 content if you're really keen.
You're probably better off sticking to questions about topics you've already learnt for now, there's not really any point trying to answer questions about content you don't know.
As for finding the time, I used to plan a week or 2 in advance when I'd be doing exams - normally after school because that's just when I had enough time, although I also had more free periods during my vce than most people do so I'm sure that helped. Do them as close to exam conditions as you can, put your phone far away and try and pick times when you're not going to be interrupted.