What is your favorite exercise for weight/strength training?
I enjoy yoga which is more associated with flexibility training.
What is a good length of time per day to do yoga for flexibility?  I would like to incorporate yoga into my exercise routine.
20 minutes per day is sufficient enough to increase flexibility.
Any other benefits you get from doing yoga?
Hatha yoga is a type of yoga that emphasizes physical exercise to master the bod and the mind.
That sounds like that will help with breathing as well.
Zen yoga
Zen yoga refers to a variety of physical and energetic practices that can be found within the Zen Buddhist tradition, and increasingly taught in the West. Some Zen temples include a taiso (exercise) period, often early in the morning, including yoga-like postures, quick repetitive exercises, and/or more flowing exercises reminiscent of Tai Chi. These exercises are designed to open and unblock the body in preparation for sitting meditation, develop a deeper awareness of the body, and as an opportunity to practice "becoming one
which is what cardiac yoga focuses on with breathing techniques as well as flexibility and concentration on specific areas of the body.
An example of cardiac yoga would be doing the sun salutation pose while sitting on a chair. After improving strength the person may stop using the back of the chair to support themselves and eventually, by gaining confidence and strength they may be able to complete the exercise standing without support.