As many have stated the stock solution keeps for long but things are different for the working solution (1+1). I usually adjusted this working solution with the necessary to fill 1500ml of liquids and keeping it for ever. But yesterday I found out that I gained truly one dia by working with a new freshly made working solution (1+1) !
So the result between f11 x time in older working solution were equal to f16 x equal time in fresh working solution.