Hi Laura,
We did a Tuscany cycling tour this summer, and although not in the same area, I'd say the longest climb we did was approximately 5 miles at 7-10% grade. Keep in mind we were on an "advanced-beginner" tour, so if you are on an intermediate or advanced tour you could face more climbing.
Thorn is right in that most of the hills are rolling. Most of our climbing was done as the end of the day in order to get to our lodgings. In Tuscany, most of the cities are atop hills!
I hope you have a wonderful time. We sure did!
P.S. If this is a self-guided tour, will you have your routeslips/maps ahead of time? If so, I highly suggested plotting your routes out on something like Map My Ride or Routeslip. Although I've found both a pain to use, if you have patience, you can plot out our route and get climbing info as well. I was able to do it for our trip after we got back home - it was fun to see the stats of what we did.