I too mix 5 liter batches of Xtol with distilled water: fill 3 glass bottles to the rim, and the rest I divide over smaller glass bottles with either 150 or 500 ml stock, and I fill these bottles to the rim with deminiralised water. So when I either process a 35 mm or a 120 film I take the appropiate bottles, and dilute it to either 300 or 1000ml for Xtol 1:1 in my patterson tank.
The oldest stock used in this way is 2 years old: no problems.
The only problem with Xtol is that it does not change colour when its dead.
Just to be overly sure I dip an exposed piece of film in the mixed developer, and see if it slightly blackens..
Best,
Cor