If you want to do well, 10 hours per week per unit is recommended, that's usually 40 hours a week, including class. I have found less than 5 does pretty well.
First year is pretty easy from what I've experienced so far.
I agree with thorrnydevil. 1st yr is pretty straightforward - as long as you keep up to date with readings etc. This will make sure you are not stressed out during exam time which quite a few 1st years often are (at least from what I observed while at uni).