"The theme park world" nature of the game means that we can't exactly tell you how much content you have left because we don't know which bits you've done and which bits you haven't; they mostly all sit in their own little isolated bubbles ten feet from each other, and you do them, and then move on... So how much you've got left will depend heavily on how many of those bubbles you've poked.
If you've thoroughly explored the surface map and have done everything you can find top-side, and have explored most of the equivalent space underneath, then you're close to done, most likely. The duergar are at the point where you can formally 'end' the act - basically right where you are, in fact, but that won't necessarily mean you've explored everything and done all of the other bits and pieces that sit in isolation on their own.
