From a reader:
Retrieving some books from storage this afternoon, I came across a bottle of holy water that I filled at the shrine of Our Lady of Walsingham a *long* time ago — more than 20 years ago. The bottle is tightly sealed, and there’s no obvious growth in the water. That is to say, I don’t see any readily apparent reason why it shouldn’t still be used.
Are there are any rules, guidelines, or traditions about how long holy water may (or should) be kept?
I don’t think there are any guidelines beyond common sense.
That said, some people have added Holy Water in the preparation of foods. I wouldn’t do that with old Holy Water. Frankly, I wouldn’t do that with any Holy Water that I hadn’t just blessed. As a matter of fact I have never added Holy Water in food preparation. I am sure there is something witty to be said about this, but I can’t get my brain into that gear at the moment.
It might be of interest to note that in the traditional form of blessing Holy Water, with the older Rituale Romanum, exorcised and blessed salt is added to the water. The salt can act as a preservative, reducing the possibility that algae might grow in the water.