Never completely. Spent material is also casing for the actual fuel, for instance, which can't be reused. In the fuel itself, there is some uranium and plutoniu

Never completely. Spent material is also casing for the actual fuel, for instance, which can't be reused. In the fuel itself, there is some uranium and plutonium which can be reused, but there are also atoms produced by the fission which are not useful. For instance, a possible output of an uranium atom splitting could be one Krypton atom and one Baryum atom. Both are radioactive, and the path for krypton to a non-radioactive atom includes one step with a half-life of ~10^6 years. There is also waste from the plant itself. When you decomission a plant, the vessel steel is going to be radioactive and needs to be stored safely. There are plenty or other waste types that can be stored more or less safely. Some can be reused, some not.