I think that the shortest you should spend in any city is 3 days. 4 days seem "just about right". You essentially lose half a day each time you transit from city to city, checking out and in to hotels. For a 23 day trip, I'd try to take in no more than 5 cities at the most. Give yourself the time to relax and enjoy the sight and sounds, and minimize/reduce the stress that comes with unexpected schedule snafu's.

Enjoy Europe!