I finished the game, it took me 6 hours, 2 hours per chapter, around 15 minutes per mission. A real blast, enjoyed my time with it a lot. Some later missions have a bit messy layout and without a map or any kind of guide feature it took me a while to get my bearings. I'm certain that if I knew all the maps perfectly well I could have shaved off at least an hour which was spent backtracking and looking for the right doors. The first few missions are the best with their linear structure, the later DOOM like levels were a bit meh at times. Still, the gameplay is excellent throughout.