George Muller a man of prayer and man of faith. A definite prayer of importunity follows:

In November, 1844, I began to pray for the conversion of five individuals. I prayed every day without a single intermission, whether sick or in health, on the land or on the sea, and whatever the pressure of my engagements might be.

Eighteen months elapsed before the first five was converted. I thanked God and prayed on for the others.

Five years elapsed, and then the second was converted. I thanked God for the second, and prayed on for the other three.

Day by day I continued to pray for them, and six years passed before the third was converted. I thanked God for the three and went on praying for the other two.

These two remained unconverted.

"The man to whom God in the riches of his grace has given tens of thousands of answers to prayer in the self-same hour or day in which they were offered has been praying day by day for nearly 36 years for the conversion of these individuals, and yet they remain unconverted. But I hope in God, I pray on, and look yet for the answer. They are not converted yet, but they will be."

This was the faith that carried him through every straitened place. He met emergencies by asking and in due time God supplied whatever the need might be.

Those prayers? You ask. In 1897, those two men, sons of a friend of Mr. Muller’s youth, were not converted, after he had entreated God in their behalf for 52 years daily. But after his death God brought them in the fold.
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George Muller by Basil Miller pg. 146

Our dear Brother, Mr. George Muller, has on his "prayer book" the name of a Brother for whom he has prayed, I think I heard him say, some 36 years. That was some years ago, so it must be a longer time than that, now, unless, indeed, the prayer has been answered. But he has the inward persuasion that this person will yet be brought to the Savior's feet and, therefore, he daily mentions the case before God in prayer. By the way, he tells us of a very admirable plan of his for booking his requests in prayer and marking them off as they are answered—and those that are not answered he lets stand until, in process of time, he finds that some of them were not proper requests and he puts that against them. But he finds that God does hear prayer and he likes to keep a record of it. If we did the same, we would have much more holy commonsense confidence in God, and our praying would be a more business-like matter as, indeed, it ought to be. But do not say to yourself, "I shall pray just so long for this thing." If what you are asking for touches the Kingdom and the Glory of Christ, persevere in the prayer with this text to encourage you, "Men ought always to pray." - Charles Spurgeon

"Here is the great secret of success. Work with all your might; but trust not in the least in your work. Pray with all your might for the blessing of God; but work, at the same time, with all diligence, with all patience, with all perseverance. Pray then, and work. Work and pray. And still again pray, and then work. And so on all the days of your life. The result will surely be, abundant blessing. Whether you see much fruit or little fruit, such kind of service will be blessed... Speak also for the Lord, as if everything depended on your exertions; yet trust not the least in your exertions, but in the Lord, who alone can cause your efforts to be made effectual, to the benefit of your fellow men or fellow believers. Remember, also, that God delights to bestow blessing, but, generally, as the result of earnest, believing prayer." - George Muller