It depends a lot of the quality of the (tap-) water. So in doubt use destilled water for Xtol and you will be fine for at least 6 months of a 5 ltr. stock. Dividing in smaller bottles will of course prolong the life time however over 9-10 months is not very wise with this type of Ascorbic Acid developer. You can also use the same type developer from Foma: Fomadon Excel W27 in the 1 ltr. packing. A tick more grain, for the rest (almost) the same.