Well, we are told that you are meant to do as much time at home as you do in lessons, so for me its meant to be 4 or 5 hours of work per week. However this is very much doing what you feel you need to do, rather than any set work. I can't really speak for human biology or psychology, but for maths, how much work you need to do on a given week depends on how well you understand that topic. Sometimes you might not need to do any work, if the topic just clicks and you understand it straight away. Other times, you might have to spend hours trying to get your head around it. So I imagine its pretty similar for the other two subjects, theres no set amount thats going to get you what you want, you just need to do what you feel is best and will get you the grade you want to get.