I have always used distilled water when servicing my batteries. My question is: is there such a thing as distilled water getting too old to use?

I use a Dual Pro on-board battery charger, which I keep hooked up when ever the boat is at home. I check and usually add water to my batteries at least once a month in the warm months and every other month or so from October to March. At the rate I am using it, a gallon of distilled water lasts well more than a year and I am wondering if I should get new distilled water or keep using the stuff til it all gone. I know its not a lot of money (especially compared so some of the other things I spend money on) but I hate to throw something away that is still usable.

Thanks for any input.

Grumpy