First you need to answer the question of what you are looking to do on this trip. I’m a cultural/history kind of person, and I spent 7 days in Rome and barely scratched the surface of what I wanted to see (granted, by then I was a little sick of the city). I suggest that in order to decide how long you wish to spend somewhere, you need to research what it is that you want to do there. Some of these cities may warrent more time because of a particular interest you may have in their attractions.
 
Personally, Venice should be 2 days max. It’s beautiful and interesting, but there is very little to actually DO there.
 
Also, for Florence, if you are planning to take day trips from there, you should allow yourself enough time to explore the city itself. Florence was my favorite part of my Italy trip. Make sure you give yourself at least 3 days of actual city time to explore.

Slayer,

What time of the year are we looking at? 

That should determine the itinerary, in my opinion.

That’s too bad.
 
If she thinks Milan is more worthwhile than, say, the Amalfi Coast or Sardinia…[&:]
 
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P.S. Southern Italy is the following regions: Campania (where Naples is), Sicilia, Molise, Puglia, Calabria, Basilicata, and Sardegna. 
 
 

Cinque Terre isn’t a day trip IMO
I’d spend a minimum of 2 nights there

a sort of obvious stopover with so much time is Pisa

I agree with the luv_the_beach‘s suggestion the time of year will affect travel plans

perhaps visit Mt. Etna?

No, no that she has no interest in Naples, other southern areas just not on this trip.  We are meeting her in northern Italy so she has to go there but we aren’t really gonna spend much or any time in Milan but rather go to Lake Como or nearby towns.  The time period wil be late September through early October.  Im thinking that she can base in Florence and drive around throughout that region spending 2 or 3 days in the city and then a day going to Cinque Terre and a half day at Pisa if she wants.  I agree that Venice is not worth the time past a day or so. 

Ah, she’s flying into MXP, that makes sense.
 
Your plans sound good.

Last year with my school group we visited Paris then Nice, not that france is on your itinerary but i want to give you and idea my path forthe trip. From southeastern France we headed in Northwest Italy. We were riding on a bus and we first went to Piza for like a 2 hour stop. The only thing there is the tower and a couple surrounding building which is all on very nice grass. The rest of the city didn’t look to great to see. From there we finished the day in Florence and stayed there another day. Florence was great. Def. visit David, and go up into the top of the dome on the cathedral. Its a longish walk but definately worth the view and maby 5 euros. Then the next day we headed to Rome but took a day stop in a small town called Asisi. The town was ontop of a big hill and was the most beautifull part of my trip. Its home to St. francis and is most definately worth the day stop. There is even a old castle on top and its like a 10 minute walk to it and you can see the whole surrounding area of mountains and hills. Def. recommend stopping in Asisi.