How to see the best of Paris, whether you have half a day or a full week
Although I did once fly into Paris, head downtowh, spend three hours in the Louvre, and then got on a train to somewhere else (I think it was Prague), intentionally scheduling anything less than three days in Paris is folly. Four or five days is more reasonable.
I know that kind of thinking is tough on a tight schedule. However, if you're flying into Paris on a longer European trip, remember that upon arrival in Europe it's wise to plan one day of officially doing nothing to recover from jet lag—so giving Paris five days in your itinerary won't be so hard.