mino, I couldn't tell you "how hard" but I can tell you that it is very, very common for athletes at Williams to excel academically -- in the sciences and everything else. These are high energy multi-faceted kids and many are involved in the arts -- theater, music, etc. -- along with their sport or outdoorsy activity.
Williams encourages participation in athletics and the arts. Academics are demanding sure, but they manange to fit in everything, including a party or two.
My only qualifier would be that the personal satisfaction level depends on your GPA expectation. If you're targeting a 4.0 you may be disappointed. If you're okay with a mix of A's and B's you'll be fine with a life outside of the classroom.