Can a protection prayer said by a deceased father for the protection of his son be answered decades later? Will the prayer power be degraded, but still sufficient to deliver the goods decades later, but maybe not centuries later when the son is reincarnated?

There is a general decline in applicability, if not doability, of a prayer. As we have discussed elsewhere with you about prayers launched but not reinforced through repetition, if it is for a powerful reason with a large intention behind it, it might well persist beyond the life of the person in the living because their intention still matters. It was given by them by virtue of their standing as a divine being, an extension of Creator, and has worth and value in its own right and will not matter where you are and what you might be doing even if you are continuing to be in the living or back in the higher astral realm. So such a prayer can be acted on for something of great importance like protection and safety. It can even extend into another life and this is true of many human thoughts and commitments such as vows, oaths, and agreements that are made as a solemn promise. This is the power of intention, to be sure, it is quite real and this is why you need to take life seriously and especially such interactions because they do go on record and there may be karmic consequences from failure to live up to your end of a bargain, but there can also be adverse things that might happen if the intention is not lofty, and this is where people get into trouble with high-level psychic attacks that might even outlive them in their reach, destructive force, and negative karmic consequences that will return to them when they return into an incarnation and that negative karma catches up with them. So life is a serious matter and has serious energy supporting it and is more wondrous than people give it credit for. So you can count on your long‑term objectives being honored, but keep in mind the best policy is regular reinforcement, so the energetic equation favors action more than inaction.