No temptation in the life of intercession is more common than failure to persevere.
We begin to pray for a certain thing; then finally, when we see no answer, we cease altogether from prayer concerning it. Not praying it through to a successful answer has formed a habit of failure. To faint is to fail.
How long shall we pray? Do we not come to a place where we may cease from our petitions and rest the matter in God’s hands? There is just one answer, pray until the thing you pray for has actually been granted, or until you have the assurance in your heart that it will be. Only at one of these two places dare we stay our importunity. So we dare not stay our prayer until the answer itself has come, or until we receive the assurance that it will come.
In the first case we stop because we see. In the other, we stop because we believe, and the faith of our heart is just as sure as the sight of our eyes. This is the faith of God, within us. More and more as we live the prayer life, shall we come to experience and recognize this God-given assurance, and know when to rest quietly in it, or when to continue our petitioning until we receive it.
“The Practice of Prayer”