The fiancee and I spent 5 days in Rome, 4 in Florence, 3 in Milan and 5 in Venice for the Italy segment of our 6 week trip in Europe.
I'd love another day or 2 in Rome. Well, in all of the above really.
Dress well, don't look like a tourist, do and eat what the locals do. You'll have a splendid time indeed.
Next time we head back to Europe, I hope to get a car and drive along the country side and explore the country that way.