Wireless subscribers are holding on to their phones for the longest period of time since J.D. Power and Associates began tracking the figure in 1999, the market research firm reported today.

The average time customers kept their feature phones rose to 20.5 months this year, up from 17.3 months in 1999, the company reported.

Kirk Parsons, senior director of wireless services at J.D. Power and Associates, said in a report that the increase could be attributed to the state of the economy.

“Typically, when upgrading to a new cell phone, there’s the added expense of either subscribing to a more expensive service plan and/or incurring termination fees when switching service providers,” Parsons said. “Today, consumers are really watching their wallets, and any added discretionary expenses are being considered more thoughtfully than in the past.”

The research firm said steadily-increasing monthly bills have contributed to a hesitancy to buy new devices. According to the report, the average reported monthly wireless bill is $78 in 2010, including federal and industry service taxes and fees, compared with $69 just three years ago. The rise is mainly due to the addition of data-related services, increased use of services like text messaging and added fees and taxes.

J.D. Power also announced that Apple ranked highest in customer satisfaction among manufacturers of smartphones for the fourth consecutive time, with Motorola and HTC coming in second and third, respectively.

LG ranked highest in overall wireless customer satisfaction with traditional handsets for a fourth consecutive time, with Sanyo and Samsung following in the rankings.