The same variables that affect the cost of learning to fly will affect the time it takes to earn your certificate. The FAA has established the minimum number of flight hours needed to obtain a certificate. Under Part 61 of the federal aviation regulations, the minimums are 20 hours for a sport pilot certificate, 30 hours for a recreational certificate, and 40 hours for a private pilot certificate. Some schools operate under an alternate regulation, Part 141, which provides more FAA oversight, more rigid schedules, and more paperwork. The added requirements allow them to reduce the minimum hours of private pilot training to 35 hours. However, many schools believe that a true average flight training time for a private pilot is between 50 hours and 60 hours, whether the school operates under Part 61 or Part 141 schools. Others believe that 68 to 70 hours is the more likely average. These flight hours can be spread over a time span of several months to a year or more.
http://www.flighttraining.aopa.org