Q-Do ceremonies performed for the dead actually benefit them? If so for how long?

Ma-Yes, they do benefit them, and for always, for even if the departed person were reborn, these prayers, being spiritual in nature, would not fail to uplift the soul, which after all, never dies, but remains the same from life to life, continually progressing in its onward march towards God, till it finally merges in Him and thus attains its liberation--Mukti, which is Supreme, Everlasting Bliss.
Then knowing the history of the grief-stricken couple who had put the question, She related anecdotes of bereavements which resembled the one they had suffered, and told how in one case the departed child was seen by his pining mother in a dream extremely happy in the company of thousands of other children, and how the deep yearnings of the mother caused the soul of her dead child to take birth again in her womb very soon afterwards, which disturbed his rest and retarded his progress. Grief is thus shown to be most harmful to the beloved ones whose loss we mourn.
The other case She related was that of the death of an only son. The mother, seeing the father unconcernedly smoking his hukka in a corner, piteously wailed and, with abundant tears accused him of hard-heartedness, to which he replied: "Do not think me to be indifferent to my son's death, but just now I am puzzling which son's death of all the dozen's of sons I have had in the last eighteen lives I should mourn." This gave great comfort to the couple concerned, which made them look upon Ma as a saviour-mother in whose lap they could always take shelter from the stormy blasts of life.