Welcome to the boards!
Generally, I prefer spending at least 3 days in a city-- preferably more. It's enough time to unpack, wander around and get a good solid feel for a place.
Of the cities on your list I've only been to Vienna, but I can't imagine that you'll have anything less than a smashing time!
The best piece of advice I can give you at the moment without knowing what your other concerns are is this: Try not to limit yourself to a strict itinerary. It's very possible that a city and/or its inhabitants
will steal your heart and you'll want to stay longer. You can make hostel reservations a day or two in advance, which will give you both peace of mind and flexibility in your itinerary.
Don't worry-- you'll have an absolutely fabulous time!