I find that one night is usually enough even in the top end ryokan (I stayed in Elizabeth Taylor's old room at the Hiiragiya in Kyoto ) The only problem with ryokan is that you're pretty much held hostage from about six o'clock when dinner is served. This is fine if you are at an onsen or resort ryokan, but can be a little frustrating if you are in a city like Kyoto or Tokyo. It's worth spending more on one of the more expensive ryokan as you are ultimately paying for the food and in a country like Japan, you get what you pay for! In Kyoto, the Hiiragiya has a reasonanly priced bekkan (annex).