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.
Charles Poliquin invented this glute tri-set for alpine skiers, but it works great for bodybuilding purposes, too. It's also a good routine if you're an ass model, but there aren't too many of those around.