Rome wasn't seen in a day
How long should you spend in Europe's major cities
The chart to the right gives you an idea of the bare minimum reasonable number of days it takes to "do" Europe's major cities: to settle in, see the major sights, get a taste for the place, and maybe make one day trip. You could spend less time, but you'll be missing big chunks for sure.
Keep in mind, this is a minimum time frame, and I was brutally honest in these assessments. It drives me a little bit nuts to say that you could, conceivably, visit Seville in a day or two. Personally, I'd rather spend a week there, at the very least. And for someone who has spent nearly four years of his life in Rome, it breaks my heart to admit that a tourist seeing Europe at a dead trot could make do with just three or four days in the Eternal City.
Also remember to add on at least an extra day for each major excursion or sidetrip you'd like to take, such as Bath or Stonehenge from London, Versailles from Paris, or Toledo from Madrid.
Of course, you won't run out of things to do if you stay longer anywhere, and I highly recommend more days than this minimum for some cities in particular—Rome, Paris, and London come to mind.
Most of these cities you couldn't exhaust in a lifetime of diligent sightseeing. Like I said, I've actually lived in Rome for around four years, and I still discover at least one new and indescribably wonderful thing every time I visit.
Beyond the Big Cities
Many travelers returning to Europe for the second or third time are discovering that there's so much to see outside of the major cities that they're forgoing trains and hotels for rented cars and villas. They go hill town-hopping and explore one tiny corner of Europe at a time, traveling at a leisurely pace away from the crowds and pressures of a rigorous sightseeing schedule in the big city.
I wholeheartedly endorse that plan, but on a first visit you'll probably want to pack in as many major cities and sights as possible—and there's nothing wrong with that. The whirlwind tour still remains the best first-time visit. It gives you a sampling of everything, so you'll know which bits to come back and explore in more depth. It also gets all the "required" sights out of the way, so when you return (and you will come back), you can concentrate on discovering Europe's lesser-known sights and facets on your own.
Trust me: You won't exhaust Europe in one trip, and it's so easy to come back once you realize how effortless and enjoyable travel here is. I've been returning for 20 years—sometimes spending six months in a single province, sometimes hitting eight countries in three weeks—and haven't even come close to seeing all I want to see.
To help you along, I've whipped up a batch of perfect itineraries for spending a week or two exploring Europe or just one country, and from one to three days in the major cities.