How much business logic should be allowed to exist in the controller layer?

Question

Sometimes we have some business logic represented in the controller code of our applications. This is usually logic that differentiates what methods to call from the model and/or what arguments to pass them.

Another example of this is a set of utility functions that exist in the controller that may work to format or sanitize data returned from the model, according to a set of business rules.

This works, but I am wondering if its flirting with disaster. If there is business logic shared between controller and model the two layers are no longer separable, and someone inheriting the code may be confused by this unevenness in location of business logic related code.

My question is how much business logic should be allowed in the controller and in what circumstances, if any?