Prayers Should Be Concise
But the Christian's prayer is easy, and it does not cause hard work. For it proceeds in faith on the basis of the promise of God, and it presents its need from the heart. Faith quickly gets through telling what it wants; indeed, it does so with a sigh that the heart utters and that words can neither attain nor express. As Paul says (Rom. 8:26), "the Spirit prays." And because He knows that God is listening to Him, He has no need of such everlasting twaddle. This is how the saints prayed in the Scriptures, like Elijah, Elisha, David, and others - with brief but strong and powerful words. This is evident in the Psalter, where there is hardly a single psalm that has a prayer more than five or six verses long. Therefore the ancient fathers have said correctly that many long prayers are not the way. They recommend short, fervent prayers, where one sighs toward heaven with a word or two, as is often quite possible in the midst of reading, writing, or doing some other task.
(What Luther Says - para 3464, pp. 1085)