As Paul mentioned, three formal nights on up to 10-night cruises, more if yours is longer than that. Figure roughly 1 formal night for every 3 nights of cruising (just base that on an average), so a 12-night cruise will normally have 4 formal nights (at least ours did).
__________________ 46 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!