Dr. Martyn Lloyd-Jones writes,

“Prayer should be going on throughout the day. Prayer need not of necessity be long; it can be brief, just an ejaculation [‘shout out’] at times is a true prayer. That is, surely. what the Apostle Paul means in his exhortation in 1 Thessalonians 5:17. “Pray without ceasing.” …As you are walking along a road, or while you are working in your study, you turn frequently to God in prayer.”

(from his book Preaching and Preachers, p. 170).