I have a coop game with my wife too. We have about 40 hours on our clock too, and we just yet entered the Underdark.
At that pace I expect our game to last for years, because in my "normal" run I am at 170 hours and still have plenty to do.
You can of course do it faster, by lowering diffiCulty (we did) and by just following the main quest. But much fun lies in exploring the world and finding the many secrets.