Arx is one quest chain that shouldn't take more than a few hours if you know what you're doing.
That's the hard part though, knowing what you should be doing. You can only advance through exploration (or searching for an answer) as the entire progress is all about connecting the puzzle on your own with minimal hint. The NPC just tell you where you can possible go through something like "MAYBE THEY'RE HIDING IN THE SEWER" or "Yeah, he might have a hidden vault".
