As little as  5 minutes has measurable effects,  and longer is even better – it can reduce brooding and obsessive worry, symptoms of ADHD, and memory loss [think dementia]. The effects are even stronger when you walk while your electronic devices are turned off.  You can also enjoy stronger focus, better memory, and creative problem solving, especially if you walk/hike near water.  So you can enjoy walking outdoors, while reducing stress and improving your focus and contentment.
See the research summaries at
http://www.wimp.com/what-
