How often must someone do stretching exercises for back pain?

As often as you can. I don't know what is causing your back pain, but it wouldn't change my response very much. Back pain is most commonly caused by posture, and muscle imbalance leading to spasm. Exercise should help promote awareness of your body and your posture, strengthen what you need to strengthen, and stretch what you need to stretch. Yoga is ideal for this. Body awareness will help prevent re-injury, too.