Well, there isn't really any sort of limit; however, rating each post only gives you one point/rating. Therefore, to really do anything substantial by rating others would take a good amount of time, and would probably be better served by just being active in the forum.
On the other hand, if a user abuses the forum by making continual random postings, etc, we are able to deduct points as necessary and/or lock the account. I'd like to believe that most everyone on the forum is here because they have genuine questions/input, so I don't foresee this being an issue.