Read up on the Hill sphere:
http://en.wikipedia.org/wiki/Hill_sphereHow much mass you need to retain rings isn't an absolute quantity. It depends on distance and size of other masses. A 250km body in Earth orbit couldn't retain its own rings because Earth's gravity would dominate the system. But a 250km body in deep space with no competition could.
Neptune can actually maintain satellites at a greater distance than Jupiter can, despite Jupiter having over 10x the mass. This is because Jupiter has closer competition from the Sun and Saturn, while Neptune has enormous amounts of space to itself.