I actually feel like I'm studying for maths the least. Then again I like maths so it doesn't really feel like studying.
Anyway, I like to avoid the trap of counting how many hours you study since as you said before, x hours of study doesn't always correlate to y result. Rather, think about study in terms of achieving goals. Do I understand the key concepts outlined in the syllabus? Can I recall any knowledge required? (I.e. in maths formulas although I don't recommend rote learning any- you should be able them commit it to memory with enough practice. Also learn derivations for your formulas so you can gain a better understanding of them and have a way to figure it out in the exam I'm case you forgey) And can I apply this knowledge and understanding to questions in the reasonable amount of time? (As measured by doing past papers). If not, find ways of achieving these things in your own way.