Remember, your body is accustomed to pauses between breaths anyway. You can stay motionless in this state, even firing several shots, or until you begin to run out of oxygen. However, you should have finished shooting before you do run out. By focusing, you should also be able to align your sights on the target, and take your shot within 5 to 10 seconds of bringing your arms up.
If you are taking longer than 5 to 10 seconds to fire your shot, put the gun down and start all over again since you are either holding your breath too long or taking too long to focus on your sights.
This is the easiest shooting fundamental to master since your body knows involuntarily how to do it. You can learn to help the process by remembering to oxygenate first, and then leave the lungs and diaphragm in a relaxed state while shooting.
Remember if you always practice slow breathing, then it all becomes muscle-memory, and second nature any time you handle your firearm.