I think the most common answer to this question is a one-liner, yet I can assuredly say that I have done many different exercises to build more shoulder endurance and that some of them have worked very well. There are three staples to the workouts I have been doing and will keep doing to work on shoulder endurance.
- Isometric Lateral Raises
- Shadow Boxing (with or without light weights)
- Boxing Apparatus (Double End Bag, Speed Bag, Heavy Bag Intervals)
Isometric Lateral Raises is by far one of the exercises done to increase shoulder endurance - this is directly linked to being able to hold your gloves in the defensive position for longer - To perform the exercise try holding up two weights with both arms fully extended like an eagle. At first, go for as long as possible and up the time by 5 seconds every week.
Shadow Boxing should not come as a surprise. The continual application of this technique has been proven by many professionals and amateurs alike to increase their ability to box for longer. You should try to shadow box (with or without weights) before any boxing workout at least to get your perception going. Constantly shadow boxing will improve the shoulder endurance in relation to throwing punches as the expended effort not only helps the muscle but improves your technique so you will not be wasting energy in needless punches. With weights, you can try to be a lot slower in shadow boxing but make sure you make every punch count and follow up with combinations.
Boxing Apparatus has been used throughout history as not only a method of sharpening your skills but also your muscles. Speed Bag will have you continually hit a ball at head height making you rotate your shoulders and get used to them being above your head. The Double End Bag will allow you to make more efficient combinations and decrease the amount of energy you undoubtedly waste in punches. The Double End bag is about having a rhythm and technique, don't go about throwing bombs. The Heavy Bag intervals can largely improve your capacity for punches in a minute, try throwing fast but light punches for an entire round, then slow and precise ones after that round to recover.
These are techniques we use at my gym - and I personally have used to increase my capacity from being able to do about 100 punches per round for 6 rounds to about 250 punches per round for 12 rounds. Constant application and practice will naturally improve shoulder endurance. However, do keep in mind that even at the end you might not improve so much this will not be a failure because the exercises do not work but rather that you are expending too much energy in your punches that will naturally cause tiredness.
Hopefully, this will help you!
- My Boxing Gym / Personal Experience
Sorry for Non-Academic sources, but sadly not a lot of people publish great methods :( Personally I wouldn't have used many sources but it shows that some people agree with this view. THIS IS NOT THE ONLY WAY TO TRAIN keep in mind.