Zero. When negative charges leave the rubber belt, they move immediately to the outer surface of the sphere. Negative charges like to be as far away from each other as possible, so they move to the outer surface of the Van de Graaff generator’s sphere.
An old, damaged photograph of British physicist Michael Faraday, who was the first to describe an electric force in terms of a field. He also invented the Faraday Cage, which permits electric charges on the outer shell but not within the cage itself.