“The Midrash explains (Yalkut Shimoni, 31) that when Hashem decreed that Moses wouldn’t be allowed to enter the Land of Israel, Moses prayed 515 prayers, until Hashem commanded him to stop praying!  Why?  With one more prayer, Hashem would have been obligated to rescind the decree.

We learn from the above Midrash that when we meet the required prayer quota for a given request, the request is filled!  As long as Hashem doesn’t command us to stop praying, we can – and must – continue to pray until our prayer is answered.” ~ The Garden of Emuna