Good question, star, and the answer to that is as individual as every traveler's style.
I guess one way to know you're "cramming too much" in, would be to look at the percentage of your time that is going to be spent in transit from one place to another. And add to that total, the time you need to allot to locate your hostel, get checked in, find your way around the surroundings, and so on.
If you're doing all of the above nearly every day, you're going to wear yourself out. Personally -- and it is personal -- I like to spend three days per city. Sometimes, I may see a different city as a day trip from the one I'm staying in, but in the long run, it saves time and makes you feel more "at home" on your travels.
The example I like to use is how I spent 3 nights in Sorrento. One day, I saw Pompeii from there (half hour on the local Circumvesuvia train, if I remember right); on another day, I saw the Amalfi Coast; on another, hmm, I don't remember! Anyway, you get the picture!
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"Life is not measured in the number of breaths we take, but by the places that take our breath away..."