Hallow and Unhallow (in D&D 3.5e and Pathfinder) allow a second spell (from a specific list) to be fixed to the site, and has a duration of 1 year.
Dispel Magic normally has a duration of instantaneous, and therefore its rules are described based on the assumption that it only functions when it's first cast.
So how does Dispel Magic work when it's given a duration? Is it cast once on valid targets that enter the area, or is it continuously recast each round? If the check fails to dispel, is the specific target creature or effect immune for a duration (such as 24 hours)?
I would understand if Hallow and Unhallow stated that they cast the specified spell on any creature entering the area, but they don't; they say only that the spell acts as if it had a duration of 1 year.
Best Answer
The Dragon #338 Sage Advice column "Official Answers to Your Questions" includes these exchanges:
(Links mine.) The Sage at the time is Andy Collins. These exchanges are repeated in the D&D Frequently Asked Questions (82-3), with which some folks have issues. While in general such rulings can be controversial, these rulings make about as much sense as anything else, and the ruling have the added advantage of actually coming from one of the revised game's designers rather than, for instance, the dude who was answering the phones that day (a risk one takes with some of Wizards of the Coast's customer service rulings).
A quick search of Paizo's site revealed no immediate and stunning insight into or changes to the operation of the spell hallow for Pathfinder, so unless contradicted by a later designer (in which case I'll happily update this answer), these rulings seem equally functional in that game.