I think that people will be active up to that point, but after that it will have no effect. Replies does not count, well that means that you are asking people to make meaningless post's.
If it should have any effect you would have to include replies and demand 2-3 post or replies every month.
But even then, trying to FORCE people beeing active on a forum is just stupid and meaningless. Only a good community and interesting debates will ensure that.