In terms of absolute time, selenium toner lasts virtually indefinitely. However, in terms of the number, sizes and kinds of prints you run through it, you will find its effectiveness can wane rather dramatically, taking impractically long to your desired state of toning. Impurities can produce a scum which may adhere to the surface of prints in toner left to stand for long periods, and precipitated silver in well used toner can muck up the bottoms of trays.