Q: Why is 20 minutes the suggested time for contemplation? Historically has that always been the length? Any particular reason for 20 minutes? Can it be shorter?
A: I answer your question from several different directions. When I was in seminary and we gathered for our spiritual exercises as a community, the tradition was that every aspect of the exercises was for 20 minutes. In Thomas Keating‘s book Open Mind, Open Heart on page 37 of the original edition, he says “20 to 30 minutes is the minimal amount of time necessary for most people to establish interior silence and to get beyond their superficial thoughts. You may be inclined to remain longer. Experience will teach you what the right time is.“ I also know that the suggested time for Transcendental Meditation is 20 minutes. It is also recommended to do aerobic exercises for 20 minutes. The one universal principle coming from all the traditions is DO IT daily. Contemplative Outreach recommends that you pray the prayer twice a day, once for maintenance and once for transformation.
You also asked if it could be shorter. Of course, you can shorten the prayer time if necessary. But be vigilant of the human condition and its various strategies to avoid transformation and deeper relationship with God. The grace of God and your own intention will assist you in practicing the recommended 20 minutes twice a day.
I hope this has been a help to you. If you have any questions and want to discuss this further just let me know.
Blessings, Fr Carl.