You could spend a lifetime looking at every work of art in The Louvre. When I visited the Louvre, I was one of the first in line, since I followed Rick Steve's advice, and and used his tour in his Paris guidebook in order to see the highlights. I would recommend his tour, and his Paris app, which I used to guide the tour inside the Louvre, that I downloaded the app from his website onto my ipod. You will be shocked when you see that the Mona Lisa is smaller than you think, especially with the hoards of tourists, and you will also notice it is behind bullet proof glass. Enjoy one of the best museums in the world, and do allow yourself at a minimum a half a day, but you could easily spend the entire day here.