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.
related posts
You can't lift a heavy bar with a weak-ass grip. It's the number one strength leak. Pull more with this forgotten method.
If your hypertrophy gains have stagnated, then it's time to shake things up. You need one of these advanced training methods.
Ian King is the antithesis of what you may think a strength coach would be.
Don't do more sets and reps than you need to. All it does is eat into your recovery and slow down gains. Details here.