Was in the area for a concert, and came here during the afternoon to kill some time. I had no idea the museum was this big. I was there for probably two hours and barely got through one floor, let alone three. The guy at the coat check (which is free, BTW....very nice in a city where it seems everything costs something) told me that the floor space is roughly equivalent to the space in the Empire State Building.
If you really want to explore and delve into some detail on the exhibits, you're gonna need all day. Plenty of food options in the building itself, but be advised it ain't cheap (I bought an antipasto salad and a Colombo coffee for $23, and that was one of the cheaper options). Food vendors set up outside the front steps know this, and those options don't seem to be a lot cheaper.