if homework and assignments counts, a fair bit
one day (a week or two before the SC) I did a whole day of geography as our teacher hadn't taught us a thing (literally) and another day I did half a day geography, half a day maths (it worked well for maths (topped the class))
as for english, history and science, not as much as I should of... but worked hard in class for all of these
so, i reckon sufficient study/homework time is 2 hours per night as long as your are working hard at school and do all homework and assignments too...
if not, a weekend of study before a test wouldn't go astray