I'll comment in terms of chemistry and biology. The recommendations that you have gotten about 6-7 hours a week sound pretty reasonable if you really want to master the courses (which of course you do)! In terms of chemistry, some things you really want to master are ions and equations. Just like you cannot master a foreign language until you learn the vocabulary, you cannot master chemistry until you learn the elements and ions. So get very familiar with the periodic table and really learn the names of the first 20 or so elements. Also you must memorize the names and charges of the most common ions. My intro chemistry teacher actually gave us a test on just learning the ions. Also memorize all basic equations (although in intro chem there may not be too many) and the units (meters, Joules, seconds, etc.) that go with each variable. You would be amazed how much you can figure out with simply learning units and equations.
For intro biology the advice is easy, you must learn all of the basic vocabulary terms and their definitions. This is definitely the key to basic biology. If you really learn all of the terminology, then you can learn and figure out how it all goes together to make up more complex systems.
On a side note, great job taking geography (although perhaps it is a requirement at your school). You would be amazed how poor an understanding most people have of basic US and world geography. Learning this will give you a big leg up on the rest of the crowd. Good luck next year!