During our trip, we plan to go from Orvieto to Siena to Florence to Cinque Terre. If you had 15 days to divide between Siena, Florence, and Cinque Terre - how would you parse it up?
It depends on the time of the year and what you enjoy doing.
All are wonderful, and the response is right. But we spent a whole week in Florence a couple of years ago and felt that we could have stayed longer. One night and a whole day in Orvieto allowed us to see and experience it, including the Duomo and all the museums; two or three days in Siena likewise. Cinque Terre depends on the weather and your love of hiking and basking.
Will you have a car? That will determine where you should stay.
Orvieto needs a day or less to visit
Siena needs a full day (2 nights or a day trip from Florence from early am to night)
Florence, just Florence, needs at least 2 days (3 nights min)
Cinque Terre is just hiking and R&R. Two full days (3 nights) is good.
In Tuscany however there is a myriad of famous hill towns and country side places (e.g. Chianti Hills) that might be worth a few days on their own.
We stayed 3 nights in Siena one October. Siena itself does not take much time to "see," but it is a good base for Volterra, San Gimignano, and other small Tuscan towns. The Chianti area is very nice, too. You can drive a lot of places within an hour or two of Siena and we liked it as a base. Siena is a lovely place to walk around in the evening, so if you are not in a rush, add more nights. Maybe up to 5 so you can enjoy some of the ambiance and the countryside. You will, IMO, need a car. Parking is challenging but not impossible. Our B&B offered a garage a 10 minute walk away, where we could easily exit without entering the ZTL.
You could even visit Firenze from Siena. Depends on how many museums you want to see in Firenze. It does not make a very good base, IMO, as having a car is more difficult in F.
We are very outdoorsy and limit our museum time somewhat as we are easily overwhelmed. (Two big art museums in a day will tank me.) So 3 nights in Firenze is enough. It's a place you have to go back to, I think. It's hard to "do" all in one trip.
We love the Cinque Terre and if the weather is good, there's no shortage of outings by train up-and-down the Ligurian coast. I have written about trips there several times in my blog Good Day Rome. There are many locales people don't get to in a two or three day stay. We always say when we go there, that if it rains we'll go to Genoa. We still haven't made it there as the weather has cooperated. I'd advise you to spend 5 nights in the CT if you are active people and like to explore. Get a little apartment with a view of the ocean.