Adding sodium sulfite works best when the sulfur is in a colloidal state appearing as a white milkiness. When the particles appear yellow they are much larger in size and conversion back to thiosulfate is much slower. Once fixer starts to sulfurize things get worse very quickly.
Two things favor sulfurization. The pH of the fixer and the ambient temperature where it is stored. The lower the pH the faster the reaction. Fixer should be stored in a cool but not cold place.
As I said the risk of ruining an important film or a print is just too much of a chance to take. If the film is not important then there is no problem.