Jesus told his disciples a parable to show them that they should always pray and not give up. Luke 18:1
The failure to persevere is the most common problem in prayer and intercession. We begin to pray for something, raising our petitions for a day, a week, or even a month, but then if we have not received a definite answer, we quickly give up and stop praying for it altogether.
This is a mistake with deadly consequences and is simply a trap where we begin many things but never see them completed. It leads to ruin in every area of life. People who get into the habit of starting without ever finishing form the habit of failure. And those who begin praying about something without ever praying it through to a successful conclusion form the same habit in prayer. Giving up is admitting failure and defeat. Defeat then leads to discouragement and doubt in the power of prayer, and that is fatal to the success of a person's prayer life.
People often ask, "How long should I pray? Shouldn't I come to the place where I stop praying and leave the matter in God's hands?"
The only answer is this:
Pray until what you pray for has been accomplished or until you have complete assurance in your heart that it will be (accomplished).
Only when one of these two conditions has been met is it safe to stop persisting in prayer, for prayer not only is calling upon God but it is also a battle with Satan. And because God uses our intercession as a mighty weapon of victory in the conflict, He alone must decide when it is safe to cease from petitioning. Therefore we dare not stop praying until either the answer itself has come or we receive assurance it will come.
In the first instance, we stop because we actually see the answer. In the second, we stop because we believe, and faith in our hearts is as trustworthy as the sight of our eyes, for it is "faith from God" and the "faith of God" that we have within us.
As we live a life of prayer, we will more and more come to experience and recognize this God-given assurance. We will know when to quietly rest in it or when to continue praying until we receive His answer.