I don't think it's possible -- or wise -- to generalise. The average amount of time you need to set aside for study depends on the average number of classes/seminars you have per week, on the amount of preparation required, the level of participation expected, and the subject (programmes including language study basically have no upper limit!)

As a full-time MA student, I agree with Blair's suggestion of approximately 30 hours per week (I've heard similar elsewhere), and Cheekybint's reference to credits equalling hours of study time is also very useful. I'd take the latter as a guideline for "normal" study and completion of a specific course within a MA programme, as for projects, coursework, and exam revision, the odds are you'll be ramping things up quite a bit.