Stock Solution A is good until it turns dark brown or black, and this appears to happen quite suddenly after about six months if there is only a small amount of solution left in the bottle. Full, or even partially full bottles are good for much longer, up to a year or perhaps even more. In any event my experience is that the solution is good as long as it is clear. Stock Solution B should last indefinitely.
Originally Posted by philldresser
Contamination of either Stock Solution A or B with minute quantities of the other will greatly accelerate aging and decrease the shelf life of the solutions. Therefore be especially careful when mixing the working solution to avoid contamination. I use dedicated syringes for each of the stock solutions.
However, if you are concerned about maximum shelf life consider mixing Stock Solution A in propylene glycol, a la Pat Gainer.