It depends on the design of the weapon. The simplest types take much more, than the sophisticated types. It also depends on the purity of the weapons grade material. That means the ratio between U235 and U238. U235 is the radioactive portion, and U238 is known as depleted Uranium. Enriching the material, by increasing the ratio of U235 to U238, is a very difficult process, reqiring the gasification, and separation of the two isotopes.