Awfully long, if mixed with water (rather than with HCA). I've used 2-year old ones, whilst replenishing them in the past. Nowadays I don't do that, as I don't like having to filter the precipitate, so I replace when it is exhausted. If you search for posts by Doremus Scudder you should find his technique for replenishing it indefinitely.
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.