It sounds to me like you are on the right track with set volume and workout duration. Some of my body part groupings allow me to complete the workout in closer to 20 minutes as well.
A longer workout doesn't equate to a better workout. In fact it is quite the opposite as you are finding out. Doing excessive set volume and extending the duration of your workout is counter productive to muscle gains from both physiological and mental standpoints.
From the mental side of the coin, short and intense Max-OT workouts allow you to maintain the ultra high focus and intensity levels needed to stimulate maximum muscle growth from start to finish. Focus and intensity will have a natural decline the longer you are in the gym.
Physiologically if you perform excessive sets you begin to fatigue the muscle and muscle fatigue doesn't stimulate growth, overload does. Muscle fatigue limits the amount of overload you can achieve and that's an important fact that makes fatigue muscle building's enemy because less overload or weight equals less muscle growth stimulation.
Keep your workouts intense, focus on achieving maximum overload in a 4-6 rep range and stick with the Max-OT prescribed set volume with 2 to 3 minutes of rest in between each set. This will put you in position for maximum muscle building every time you enter the gym.
Remember, more is not better when it comes to set volume and workout duration.
