The Jewish people prayed for the Messiah for centuries before His arrival. Their prayers were finally answered, but it took hundreds of years. Ditto for their enslavement in Egypt. It took the Church over half a century to overcome Arianism and over a century to overcome the iconoclast heresy. Saint Elizabeth prayed for a child for decades before her prayer was answered and St. John the Baptist was conceived.
It took 7 centuries for the Spanish Catholics to overcome the Muslims in the Iberian Peninsula. It took over 6 centuries for the Church to finally officially recognize the sainthood of Joan of Arc. The Jewish people will finally be converted to the Catholic Faith, as Sacred Scripture makes clear, but so far that has taken almost 20 centuries, and we don't know when it will occur.

God might grant graces through Mary but he is certainly not obliged to. Proof of this is that there have been billions of "Hail Mary's" said for the end of abortion but abortion grows more and more evil.