The answer is how many hours of studying your instructor recommends for each hour of instruction. It fully depends on your instructor and their preferences. Of course don't over work yourself. Make sure to take breaks between long periods of time.
Work Step by Step
Some instructors believe firmly in the 2:1 rule, meaning that for every hour of instruction, the student should spend 2 hours studying. So for a class meeting 3 hours a week, there should be 6 hours of studying. Ask your instructor to fully understand their expectations; this information may also be on the syllabus.