Barozi on 04 September 2017
It depends on what you're doing, but personally I think 2-3 days for big cities is enough.
Most of the time "wasters" are museums, zoos or shops.
Why visit a museum of natural history in different cities? They mostly have the same exhibits.
Same for most zoos or shops, as they are the same around the world. I do however recommend shopping and drinking in Prague. It's extremely cheap there.
