Prayer is rather like going to sleep.
Thomas, putting it very bluntly, says that when we cannot go on praying without getting bored, then it's time to stop. It is easy to praise God when everything is going well, when the sun is shining and our health is good. The acid test is whether we can go on praising him with sincerity when everything is going wrong, when you are out of work and far from home, with a headache, lost in the pouring rain and no prospect of a bus for another two hours.