How many days should I spend in Venice?

Two nights. Venice transforms after the day-trippers leave around 6pm and before they arrive around 10am, and that's the city people fall in love with — so the point of a second night is the evening and the early morning, not extra sights. One full day covers the Basilica, the Doge's Palace and getting properly lost; a second gives you the lagoon islands or a slow morning in Cannaregio and Dorsoduro where Venetians actually live. Three nights is lovely and not necessary. Day-tripping into Venice from the mainland to save money is the one false economy here — you miss the entire good part.