I spent 3 days in Venice; you can't even scratch the surface in 3 days, but I'd say 3 days is about enough for one visit. If you want to see more, book another visit.

Quite a lot of fine art in Venice isn't in crowded museums; some of it's in little churches, away from the crowds, and some is in smaller private museums, away from the day-tripper trail.

I've never been to Florence. I hear that's overwhelming.