Did you know that there are two Sorrento's in Italy?
The distance between Florence and Sorrento, Campania (near Napoli) is about five hours depending on the traffic as you go past Rome. The Autostrade (highways) in Italy stay pretty far out of the city. So it isn't too bad. But sometimes they slow down near big exits.
The distance to the other Sorrento in Calabria is 8 hours.
The distance to Florence from Venice is 2 and a half hours, depending on traffic into either city.
Have a great time on your trip.