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.
There's nothing more frustrating than committing to a training program that doesn't pan out. This is equally true for an athlete who might work hard yet still falls short of his expectations and the strength coach whose biggest satisfaction comes from the great strides his athletes make.