As previous posts have said, there is no way we can tell you a specific amount of time that will be "enough." Sorry! You need to consider the exams you will be taking, your current understanding of the material that will be covered in each of those exams, and the amount of time you need to devote to each individual subject to fill in the gaps in your knowledge bases for each of those subjects.
Study tips depend upon the way in which you learn most effectively. If you learn and remember best when you read, highlight information (color coded for different subjects, maybe?), create outlines of the information to review, spend lots of time rereading texts or problems or whatever, depending on the subject. If you learn best when you hear the information, use audio tapes of your textbooks for review, read out loud when studying or talk your way through solving equations and tape yourself for listening later. Think about the methods your teachers have used that have really worked well for you, then try to find ways to use those same methods in your study time.