I saw a student that was milked for over 50.
Absolutely CRIMINAL on the part of the flying school.
Some years ago CFI's in the Melbourne area were asked to comment on their students average to time to solo. Most agreed it was difficult to generalise because of different factors including frequency of training. 15-20 hours was near enough to average.
However it is not always instructor experience as a factor. One CFI declared vehemently he would never send a student solo until he had flown at least 20 hours of dual. He said this was because in his opinion it takes 20 hours of dual to learn "situational awareness." He owned his own flying school and was the sole instructor.
Another CFI known for his propensity to delay sending his students solo until 25-40 hours, gave a commercial reason for his caution. Years back as a young instructor he saw a first solo student tip the aircraft over on its back on landing after several nervous go-arounds. After that experience he was determined not to run the risk of that happening at his flying school. He seemed oblivious to the fact that his customers were drifting away to other flying schools after they had logged 20 hours plus dual with him and had still not gone solo. The interesting part was that both CFI's described above were well experienced and operated one man band flying schools.