re: How long should you stay in cities while backpacking?

Traveling between cities can start to wear on you considering it often ends up being a 5-6 hour travel time. Even with a month, I would consider reducing the number of cities on your list. Personally I would knock out Nice, Oslo and maybe Stockholm. Those 3 cities seem to be further from the other cities you list and are very, very expensive. Copenhagen is expensive too but is closer to the main part of europe and IMO is a more interesting city than Stockholm and Oslo. Plus you can get over to malmo sweden in 45 minutes on the train if you want to say you've been to sweden.