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.
In four months I'll be a whopping 28 years old. I know a thing or two about training and all that goes into it, but I also respect the fact that there's a hell of a lot I don't know enough about, and other topics I need to know something about that I'm not even aware of!