There are many ways of doing band pull-aparts. When I use them as extra work between sets I don't count reps or time – the goal is simply to put the muscles under tension, not to do a specific number of sets and reps.
With the therapeutic variation, once tension is established simply do slow movements until you find a sore or weak spot. When you do, spend some time keeping that spot under tension until the pain/soreness decreases.
Too often we get lost in the insignificant stuff. We need to step back and look at the big picture. What are we trying to do? Grow bigger muscles and become indestructible, of course.