How long should you spend in Paris? If "a lifetime" is not an option, here are ways to fit all the best Paris has to offer into the time you have available, whether it be one day, two days, three days, or four days:
First off, don't try to pack in too much. Paris is a cultural carousel, and attempting to see all its museums, churches, monuments, and other attractions will only leave you panting from exhaustion, frustrated.
On the day you do the Louvre, plan on doing nothing else in particular. When you are tired of the museum, head out the door to stroll the Tuileries gardens or along the Seine, drinking in the ambiance of the city.
Definitely don't leave town without ogling the art of the Louvre, paying your respects to Notre-Dame, being impressed by the Impressionists at the Musée d'Orsay, and climbing the Eiffel Tower, but leave plenty of time just to enjoy the life of Paris.
In winter you can ice skate in front of City Hall (in summer the truck in sand and the plaza becomes a beach volley ball court whiel the banks of the Seine become beaches).
The French have a wonderful phrase: faire la grasse matinée, to have a fat morning; translation: to sleep in with breakfast in bed.
Stop sloughing through museums and churches before you get tired of them.
Picnic at Versailles.
Take a Seine cruise.
In short: relax. Enjoy your vacation...
...and promise yourself you'll come back.