Yeah diaries are full of shit, more like in Year 9 - 10 you do 2 to 3 hours of homework per week LOL.
That said I wouldn't organize the amount of work you do in terms of "hours" per say, but in terms of goals. I.e. for Maths, my School has a syllabus where it says what exercises we should do per week for the whole year. So one of my "goals" is to not fall behind those deadlines each week. Apart from doing the consistent work to keep up for all your 3/4's, you also need to factor in extra work for SAC Preparation (i.e. Chapter Reviews, Practice SACs), Checkpoints Questions, spending time preparing for Prac's in Chem/Bio etc. Also pretty soon it'll be time to start doing practice exams, so I'd recommend also planning out how many you want to do and plan when you're going to do them (might mean starting 2 months early if you have 2 or 3 mid years). However long it takes you to do all these things is how long you should spend studying, but if I had to hazard a guess I'd suggest for most people that's around 3 hours work a weeknight.