I would subtract a day from Venice (you can see most if not all of the tourist attractions in 2 days) and do Pisa as a 2-3 hour stop. That gives you 3 days to add back to other places. Given the time of year you're traveling the weather could have an effect and you won't be able to see as much on a given day as if you were there on a mild spring or summer day; so those extra days added back may be useful.
Part of what makes it difficult to say how many days to stay in a place is that people have different plans for how they want to spend their travel time. For some, ducking into the top museums so they can say "I saw X and Y" and seeing some of the famous buildings in a place is enough. Others want time to stroll and relax, time to savor an unexpected discovery, to allow serendipity to play a role. I'm biased towards the latter mode, but its not the only way or even always the right way. RS's signature tour might be his 21-day rumble across Europe, a sampling of the sights in many cities and areas. That's 2 nites at best in a place, often just one.
My gut feel is that many people as they start out traveling try to see a lot, but then as they accumulate trips tend to stay longer in fewer places.
