re: How long would you stand in line to see Mona Lisa?

Met special exhibits can be brutal especially during holidays when out-of-town relatives hit. Skip those, make a virtual gallery visit online, then go see a few long-term exhibits that interest you. Arms and Armor is a good choice.

Extra tip if you are in Paris, don't stand in line at the Louvre main entrance. Enter underground via the Metro station... usually fewer people.