Blymath- The length of time you spend in Paris will depend on what you want to do. Once you start planning your days you may find that in order to see the Eiffle Tower (wonder around the trocadero and surrounding gardens), Lourve, maybe a day trip to Versailles, a walking tours may of the Maris and Sacre-Coeur... oh my that is four days just right there and we have not even talked about Notre dame, Siene river cruise, the catacombs or a food tour!
Florence is in Tuscany and I would never miss a chance to see that city! Florence made a great base for us to visit surrounding cities.
Do for Rome what you did for Paris, write down all you want to see, keeping in mind open/close days. Start a pad of paper (or excel) with a line for each day/date, once you plug in travel days to get from one place to the next you may find those are throw away days, planning travel around your check in/check out times. How are you getting around? Driving? Train? Bus?