Like with paint tins, if the lid is clean and well sealed then it should last a very good while as the styrene solvent will otherwise escape leaving an unworkable putty. The hardener does go crusty after a while making mixing more difficult, but I am not sure if unopened hardener goes off, I seemto recall having a virgin sachet that had gone off. Also, some sachets are not perfectly sealed around the neck and can go off through that.