How long should I spend in the Uffizi and the Accademia to see everything? I only have one day to do the museums and want to book both tickets for the same day.
This is a bit like asking how long is a piece of string, but as a bit of guide, you will need much more time inside the Uffizi than the Accademia.
In general, both museums are more pleasant first thing in the morning when slightly less busy, but do the Uffizi first thing as it will require more energy! You have to walk up to the 4th floor of the museum to get to see most of the artworks, first of all, but the collection is quite large and you will probably find yourself spending a minimum of 2 hours there to see just the highlights. You could easily spend even 4 hours if you have the strength and want to be thorough (the rooftop terrace bar for example is a good spot to recuperate for half an hour before heading on!).
The Accademia is much smaller, more compact and its main feature is Michelangelo's David and his unfinished Slaves scultpures. The rest of the museum has secondary works by artists whose masterpieces you will have already seen in the Uffizi gallery, so if you aim to do the Accademia Gallery say in the afternoon, it will give you time to rest and do other things between museum visits. Depending on how you feel once you're in there, some people just like to see David, others like to spend time to explore the medieval gothic paintings and the musicial instrument gallery, for example.