Short answer
Ten to twelve days is the practical minimum for a first Europe trip, covering two countries at four nights each plus arrival and departure days. Fourteen days accommodates three countries. Under eight days, restrict yourself to one city. Budget four nights per country after transit, not two.
The constraint is not distance, it is the cost of changing base. Each move between countries burns a hotel checkout, three to seven hours of transit, a check-in and an evening spent finding dinner — roughly three-quarters of a day. Five moves on a six-country tour therefore consume close to four days out of twelve.
Four nights per country gives you three full days, which covers one major city properly and a half-day excursion, with slack for weather or a museum you did not plan. It is what separates a trip where you saw Rome from one where the coach passed the Colosseum on the way to Florence.
Geography lets the rule bend. Prague, Vienna and Budapest sit within four hours of each other by train and behave almost as one destination, as do Zurich, Lucerne and Milan. Pairing neighbours is how a twelve-day trip gets a third country without paying for it in transit time.
| Days | Countries | What fits comfortably |
|---|---|---|
| 6–8 | 1 | One city plus two day trips, e.g. Paris with Versailles and Giverny |
| 10–12 | 2 | Switzerland and northern Italy, or France and the Netherlands |
| 13–15 | 3 | Only if two are adjacent, e.g. Czechia, Austria and Hungary |
| 18–21 | 4 | A genuine grand tour with three nights minimum in each base |