How many days for Vienna?

Without knowing what you like, it's hard to say. I like museums, palaces, fast cars, and music including opera. Three days seem like a good taste for most cities, but 4 days did not seem enough for such a historic city. A guidebook could list things more efficiently. I'd love to return for 5 days+ and revisit everything I saw on my first trip.

Three were enough for us.

Thanks to everyone for their comments!

3 was good for me. Since it was our first city of our trip, it was more like 2. But 3 full days should satisfy

3 is reasonable, but I'd do 4 if you plan on spending a significant time in one of the excellent museums. You could easily lose a day just to one of the big ones.