How many RU attempts before TPR?

Do the birth parents get unlimited amounts of RU attempts as long as they work their plan, or does there come a time when enough is enough? I have a sibling group of 3, who went home for 6.5 months before re-entering state custody, due to multiple drug issues, neglect (educational, medical, physical, etc.). Mine is the only foster home they have been in, no moves or anything (yay!). They have been back with me for 6 months now, and neither parent did anything on their plan to get the kids back. Now BM has decided that she wants to try again and is working her plan. DCS says they have no intentions of RU with mom, even though the permanency plan says it is a dual goal of RU/adoption. So my question is, how could DCS prevent RU with BM if she works her plan? They keep telling me that their plans are to TPR since we have met the 15 out of 22 months rule, but can they terminate rights if she is working her plan? I just keep seeing the same thing repeating over and over. BM getting cleaned up long enough to get kids back, them going home, her going back on drugs, kids being removed again, and repeat. Do they allow this kind of thing to happen? What is your experience? Like I said, DCS seems to think it is too little too late, and don't think RU will ever happen again, but I keep seeing other stories on here of multiple RU attempts. I would just like to hear from others about their experiences. I have heard that if DCS is opposed to RU, then it rarely happens, so I am wondering if this is true. BTW, my husband and I would LOVE to adopt them, so we are obviously hoping for TPR. Thanks, Kim