Many good comments/suggestions here.
I think the location of the previous and next stops is a consideration as well.
Absolutely, that is a consideration for how long we should ultimately decide to stay somewhere, but it doesn't really affect the basic answer to the question of what's the minimum time to get value from our visit somewhere. Once you decide how long you decide on where you're going before and after, you can adjust your time in a given location as appropriate. In addition, a string of short stays, as you say, is sub-optimal. So probably you wouldn't want to visit all the places on your itinerary for "just the highlights."
There's also the question of what constitutes "the highlights". And does one want to really see them or just glance at them? Walk past the Colosseum, go inside it, or pay for a specialized tour?
Another good point, acraven. Maybe I need to divide that into two separate categories: "Quick pass through the place to capture the highlights" and "See and explore the highlights."
Another factor that could turn the Excel grid into a cube would be “Number of people”.
One more factor might be who you're traveling with and/or if a traveler has mobility limitations. Sightseeing with very small children or elderly parents with bad knees can require more time to take in the sights than, say, a 40's couple conditioned to hours on their feet.
Yes, very true. Maybe the best way to account for this is to add 25% (or some percentage) if you are traveling with elderly/children/group of 6 or more. Depending on the limitation, it might be best to double to recommended number of days.
My fifty cents is to measure it by the number of nights.
I always plan my itineraries by number of nights, collinsdtc. I agree this is the best way to plan a trip. But this exercise is about how much time you need to experience a place. Some places can easily be experienced with a day-trip, so that would be zero nights. Once you know how many days you need to get the desired experience in a certain place, you can determine how many nights to spend there. As an example, you could visit Siena as a day trip from Florence. So one day is the minimum for getting the highlights. I would also recommend that visiting Siena is a far better experience if you can spend at least one night so you can be there after the day-trippers have left. On the other hand, visiting Potsdam from Berlin is a perfect day-trip. I wouldn't recommend an overnight. Maybe I need one more column in my grid: "Overnight recommended" with a checkbox.
Thanks again to all for the feedback. I might actually make this grid, and your ideas will be a great help.