More to the point is how well you can remember things / how fast you are with homework. So obviously, if you're not too crash hot, you should spend more time - but if you are good, you still should be doing study anyway!
Anybody who has been through year ten will tell you that it doesn't matter - however, I do understand (having been through it myself) that of course it matters - to you!
Generally, in year ten, do all you homework, plus half an hour on any subject you're having difficulties with (eg, if it's maths, do the questions twice). Coming up to assessment/trials/SC,
Year ten is an excellent time to get good study habits - trying to suddenly 'be good' in senior year is NOT FUN. I ended up having to sacrifice three of my weekends to get an extra sixty hours in total the first term. *dies*. Of course, I'm an absolute perfectionist... I'm only a year 11, but this is the formula I recommend:
Year ten
(normal week) - 1 -2 hours a day
(exam time) 2-3 hours a day, plus 3-4 hours on sat and sun
Year 11
(normal week) - 2-3 hours a day
(exam time) 4 hours plus and 5 hour sessions on weekends
BoS recommends a total of 16 hours perweek for yr 11s
year 12
(normal week) - 4 hours a day
(exam time) 5-6 hours a day, plus full weekends (8am-9:30pm)
Bos recommends a total of 20 hours for yr 12s.
I can't say about uni, because it REALLY depends on what you choose to do. Keeping the year 12 plan would probs work best though. However, I am doing one uni course as part of my yr 11. the university recommended I spend 4 hours a week on it. I say 6. (how a person is supposed to listen to 2 hours of lectures, read 30 pages +take notes, do an online quiz and talk on a forum and four pages of homework in four hours I have no idea... - not to mention four hours it takes me simply to travel to and from and attend the fortnightly tutorials. But enough complaining from me!)
I'll end with this:
I wish I was a year ten again. Is was so easy - yr 11s seem to have very little social life between school, study and keeping healthy. So enjoy what time you have now!