I fill the fresh XTOL in 5 glas bottles at 1 liter filled up to the top!
I use XTOL replinished in my huge 2 gallon tank undiluted for a bit over a year and all was fine. With every new development i was giving 1 liter fresh developer to it after I was taking one liter out!
Now I do the same at 1:1 which is not recomanded by Kodak but for 5 month it worked also this way perfect.
But its always in a dark and cold place if not in use!
In my opinion in full strenghs it lasts almost forever with replinishing!