If you look at existing domains that include some kind of attack power or spell-like ability, it's usually usable just once per day but it scales somewhat well with level. Examples are Death, Destruction, and Charm domains. Maybe use those as guidelines. The exceptions are things like being able to turn or rebuke a certain type of creature but those are VERY circumstance-dependent and most days won't be useful at all. For instance, how many days do you run into creatures of the fire subtype? If it's an ability that's useful in a broad array of circumstances, it's usually limited to once a day.