You can stay with it for as long as you feel mentally and physically comfortable. Working mindlessly is a waste of your time and does not honor the subject. If you lose focus, stop. Rest a few moments and begin again. This isn’t a race. It's an adventure.
How long should I wait after a meal to practice my breathing exercises?
Remember the diaphragm (your main breathing pump) is stretched directly over top of your stomach. I suggest that you give your food the right-of-way for a couple hours after a full meal. Both your digestion and your respiration are incredibly complex processes. Each needs to take its turn at center stage.
How do I find a proper breathing rhythm?
Unless otherwise suggested, allow your breathing rhythm to be set by your brain. Your natural feelings take into account the ever-changing needs of your body and mind. Breathing by the numbers dictated by someone else is like painting by the numbers. . . a respectable way to pass the time, requiring only that you follow orders.
Ancient Wisdom: There is no need to push the river. It will flow by itself.
Be well. Breathe beautifully.
LINK: Review #1