In my flight school class, the average was probably about 12 hours, the first was about 6 hours, and I was among the last at 18 hours.
IMHO, the biggest influence is the courage of the instructor and the second is the continuity of training. We flew 1.5 per day, and ate, slept helicopter flying, with virtually no weather, ATC or family interference. We had 210 hours in 10 months, as well as 50 hours of instrument simulation! A poor guy who flys once a week, juggles a job and has a life should take longer.