As someone studying the HSC i have been told do each subject in 30min blocks because that allows your brain to take in the information.
For example say you get home at 4pm give yourself half an hour to relax, then study 30mins of english, 30mins of Physics take a 30min stop to allow yourself to take in this imformation, do an hour of h/w or assessments just to take your mind away from all this information. (plus it needs to be done) have dinner/ chores if you have any, then do four 30min blocks of Gen Maths Ancient History Software Desgn and Legal Studies .
This is how i do it because this is how my Mentor has told me to do it so every subject gets studied for, my assessments and homework gets done. This doesnt make me to stressed, but normally like if i have exams (like for the next two weeks) i spend a couple more 30min blocks on them.
You have to find a way that works for you.
Good luck