LONG PRAYERS: Is it not right then that we should make long prayers? Did not our Lord spend whole nights in prayer? And did not St. Paul, after his conversion, pass three days in praying? Our Lord does not object to the long duration of our prayers, but to empty forms, useless repetition of words, and mere lip-service in prayer, wherein the heart takes no part. In another place He expressly says that men ought always to pray and not to faint (Luke 18:1), and St. Paul exhorts us to "pray without ceasing" (I Thess. 5:17).
[From 'A Practical Commentary on Holy Scripture' by Bishop Knecht, D.D.]