We're not big fans of NCL food in the main dining room, so I would agree 3 to 4 nights in specialty restaurants for us when doing NCL.
We normally do specialty restaurants at least twice on a 7-night cruise mainly because we so thoroughly love the food, but also because we have to be able to tell our clients about them.
I personally feel the food in all the main dining rooms have deteriorated over the last several years and wish they would charge more money and go back to the way it was than to keep prices low and have quality suffer.
Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!