If you need rules for it, you could probably call it a ritual and adapt the rules for conjuring. By default, Holy Water is pretty simple stuff (no moving parts and such, so perhaps the base difficulty would be a mere 1. That would be for, say, a vial of holy water. Then make up a size chart similar to the time charts, perhaps something like this:
vial - pint - gallon - basin - tub - spa - pool
Each step up the chart requires an extra shift. Base time might be the length of a typical mass or service, and the time could be reduced by paying shifts. Translate the preparation options for Thaumaturgy into more religeous trappings, such as holy places instead of ley lines, blessed fonts in place of a ritual circle, meditating on the scriptures instead of spell library research, etc.
The rolls would certainly require Conviction and possibly Scolarship or Lore, and only those with the proper Template and/or True Faith powers would be capable of creating it. There might even be room for a stunt adding an 'expertise with holy rituals' trapping to Scolarship.