It just depends what you have on your plate of priorities: don't think of study in terms of hours but in terms of what you need to do at that given time. You may well find yourself consuming a whole day on a particular task because its completion is paramount (though this is generally unnecessary and inefficient, if you have planned your study schedule properly).
If you want something to take from this post then: do whatever is necessary, taking into account what is required of all subjects and prioritise.