The average amount of time people keep their cell phone has increased by 17 percent from 2009, according to a new study from J.D. Power and Associates. People are keeping their mobile devices for an average of 20.5 months, which is the longest period off time since the study began in 1999, when the average was 17.3 months. The current level of ownership does differ by brand, with the highest ownership at an average of 27.8 months, and the lowest at 17.5 months.