<p>“had been” for me usually implies a habit or action that later changed. (e.g. I had been practicing for 2 hours day, but i changed it to 3.)</p>
<p>“will have been” denotes an action in the future that is usually followed by a period of time. (e.g. On September 10, I will have been practicing for 5 months). It basically refers to the amount of time spent doing something.</p>