It's a gigantic dungeon but not all of it is detailed. There were two boxed sets and three modules. If I remember correctly, the boxed sets mapped about 3-4 levels each (most of which are generally about the size of the map you saw), and of those, only certain portions of each level were written up in full detail. Large areas of the map were left empty for the DM to develop. The first boxed set covers the upper levels, the second covers the middle levels. The lower levels were never detailed. The modules detailed some sublevels, and the best of these (IMO, of course) was The Lost Level, a dwarven tomb or temple. Finally, the underground city of Skullport is part of the dungeon, and was detailed in its own sourcebook.
So, to get back to your questions, without lots of work by the DM, it's impossible to "play the whole dungeon" and even with that work, it'd be hard to say how long it'd take as it'd vary greatly from group to group. Think of it more as a setting than an adventure, and you'll be more on target.
