As imperfect as the Rosary may be, it has been said billions, even trillions of times over the centuries. If a person prays the Rosary thousands of times in a lifetime, will they be almost guaranteed to have a Spirit Rescue done for them, even if they never had competent understanding they were making such a request? Does a weak prayer said over and over again, make it a strong prayer eventually?

You are asking in a very simple way about activities that are extremely complex with respect to the energetics as they represent many, many unique situations with the innumerable variables in play for any given individual and their particular circumstances. We can say that repetition of prayer will add further energy to the cause and is always a good thing, but acts of prayer in isolation are answered in isolation, so only through the repetition can a little more each time be done to benefit someone. So these are not added together and then acted on some time down the line to bring about the full force of their combined potential and perhaps generate a miracle. What will happen is a small benefit occurring each time, so what this will contribute and bring about is highly uncertain. It might be a person will get a small boost that will fade subsequently, perhaps rather quickly, and then get a small boost the next time a prayer is launched but that, too, will fade and perhaps no great breakthrough will result—that is one scenario. It is possible that the nature of a prayer will allow some cumulative effect, but that will depend on the intention being held. Many times people who pray the Rosary will be building up a reservoir of spiritual capital because they think of the practice in this way, sort of like climbing a mountain with each time of prayer like taking a step to further their long climb to the top where they will be close to God and a breakthrough might happen for them to benefit them dramatically in some way. If they think of it in those terms, that could well come about, because this will allow the divine realm to follow that intention and bring about a cumulative benefit after a sufficient investment is made through prayer repetition. So again, it is the intention that determines what will happen and, as the components of the prayer vary, one would have to look at each particular aspect as to the likelihood of getting a cumulative benefit that serves that particular intended and requested need. But, in general, carrying out prayer is better than not doing so. Whether one gets a single divine response and benefit or a more powerful compounding, the very best way to ensure the latter happens would be to ask for that explicitly and this most people do not think to do.