You can continue praying in this way until either (1) you run out of time, or (2) you run out of psalm. And if you run out of psalm before you run out of time, you simply turn the page and go to another psalm. By so doing, you never run out of anything to say, and, best of all, you never again say the same old things about the same old things. So basically what you are doing is taking words that originated in the heart and mind of God and circulating them through your heart and mind back to God.