People actually find 5 days worth of stuff to do in Venice. I felt like I exhausted that city in 1 day at the most.
I was going to ask the same thing, although I found one day a little short. Of course that day started in Milan, took the 8am InterCittà to Venice and had to take the 5pm InterCittà back so we only really had 6 hours or so.
I've spent an entire week in London and still tacked on a couple of days when returning from Paris on another trip. That trip to Paris was also a week. There is plenty to see and do in both cities. We took a London Walks day tour to Stonehenge and Salisbury Plains which took a day out of our London trip, and a trip to Giverny (and Claude Monet's home) which took one of our days in Paris.