Accademia, Uffizi & Medici Chapels

For the Accademia and Medici Chapels, an hour each might suffice. They are not huge places. For the Uffizi, you will need probably at least 2 hours depending on whether or not you will be using some sort of audio guide (Rick Steves). If not, then you might breeze on through faster.

Of course "leisurely pace" is different for different travelers. Rick Steves gives these times for the tours of those three places that are described in his book, Florence & Tuscany. His times do not include the time to get in, stand in line, take a break, have lunch in the museum (where offered), etc, just the time to do his tour:

Uffizi: ~2 hrsAccademia: 30 mins to see David and the Prisoners, ~1 hr if you wish to see other parts of the museumMedici Chapels: ~1 hr

Rick's times seem about right for me, but some travelers find Rick's "pace" in museums to be somewhat faster than they prefer.

This is one of my pet peeve questions! If you really want to see what is in the Uffizi, it could take the whole day, easily. If you are just strolling through, without really stopping in front on anything, a couple of hours will do it, but at least half of that time will be getting in, getting up stairs, walking down hallways and leaving the labyrinth of an exit. I think it is realistic to plan on 3 or 4 hours for even casually visiting the Uffizi. You can take several breaks, visit the rooftop cafe for some amazing views and a refreshment, etc.

Last summer we easily spent 3 hrs. casually viewing all the artwork in the Uffizi and could have spent more. It is nice to have time to just sit and admire some of the artwork like "The Birth of Venus." And as Andrew stated above stop at the cafe for great views from the rooftop. We probably spent about an hr. at the Accademia viewing David, the prisoners and a special exhibit that was there. So really we just saw the minimum there.