That is my rule, and I have followed it nearly everywhere in Europe! I find that if I have extra time, I am able to do day trips from the base, rather than adding lots of travel time to do additional cities and try to do everything. I just returned from an 18 day trip, and concentrated on the cities of Budapest, Prague, Paris. I felt like I covered those cities well (note that it was my 9th trip to Paris), but first trip to Central Europe. A day trip to Terezin (short train trip from Budapest) was my only day trip this time. When I traveled to Spain, I was in Spain for 2 weeks, with 5 days in Barcelona, 5 in Madrid, but shorter stays in Granada and Sevilla.Note that from Paris the high speed trains make it possible to do day trips as far away as Dijon easily. Sometimes, a day TOUR can be useful rather than doing trains with multiple stops. I did an organised day tour from Paris to chateaus of the Loire Valley for example.