Three days is a comfortable length for a first visit and close to the sweet spot for Paris. It gives you the Eiffel Tower, Champs-Elysees, Arc de Triomphe, and a Seine cruise on day one, the Louvre with the islands, Notre-Dame, and Sainte-Chapelle on day two, and a Montmartre-plus-Orsay-and-Marais day three that balances a hilltop village with the great Impressionist collection. You eat in three different quarters, fit both a park pause and the museums that matter, and still move at a pace that lets you sit on a terrace rather than sprint past it. What still gets cut is the Versailles day trip, which is exactly what a fourth day adds. This suits first-timers, couples, and anyone routing on to Amsterdam next. Stay in Saint-Germain: it walks to the Orsay, the Louvre over the river, and the Luxembourg Gardens, and its cafes make an elegant base for the evenings. Pre-book the Eiffel summit, the Louvre and Orsay slots, and a free Notre-Dame reservation before you fly.