I would commend a longer time in focused prayer and meditation early in the morning, perhaps an hour (the length will vary with your situation in life), and then two or three other short times later in the day, roughly corresponding to lunch, dinner, and bedtime. These may be no more than a few minutes. What matters more than the length is the intensity of the focus.

In these later times of prayer, I am not referring to the thoughts directed Godward as you walk back to work from the cafeteria or as you run to the car. These are good. They are part of praying without ceasing. Rather I am referring to a few minutes of very focused stillness and solitude, with the Bible open in front of you--or the memory serving up some nourishing text on the tongue of your soul. The aim is to call to mind a few verses and to pray that God would now satisfy your heart in him for the next part of the day and free you from sinful desires, so that you exalt Christ and love people. In this way each segment of the day (and then the night before you go to bed) is consciously dedicated to God by an act of consecration in focused prayer. It is amazing how just a few minutes over the Word at midday and mid-evening can bring spiritual clarity and power and peaceful joy to the next few hours, even in the midst of much pressure.

John Piper, When I Don't Desire God: How to Fight for Joy, pp. 160-61